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watch crystal five drops of the ethereal solution of notes on this subject are, for the reasons above named, bromine are added.

rather meagre; but I give them as they were put down "In using this test, the evaporation of ether which at the time, reserving for future observations, to correct takes place from the crystal absorbs so much heat that and enlarge them, if the matter should turn out to be the mixture is kept cool, and no generation of surplus really of interest and importance, in recognising the heat can be expected. All the articles used in this volatile oils. examination are of a volatile nature, and their evapora- ! Upon the behaviour of the volatile oils towards iodine tion is favoured by a constant change of air; it is, various classifications have been based. Tuchen clastherefore, requisite to prevent this, and make the expe- sified them into fulminating oils, in oils which dissolve riments in a place excluded from draughts.

iodine completely, and such which dissolve it but imperThe employment of an ethereal solution of bromine fectly. Zeller, from his careful investigations, divided suggested the use of iodine in a state of solution ; its them into five classes, and each of them again in various ethercal solution appeared to have many advantages subdivisions. They are as follows:over the concentrated spirituous tincture, owing to the I. Fulminating or decomposing with detonation, with solubility of oils in ether, by which property a more much heat, and the generation of violet and yellowish intimate connection and a quick and sure reaction would red vapours be promoted. I have employed a concentrated ethereal (a.) Quick and violent fulmination, with mostly violet tincture of iodine, which was prepared by adding to vapours. Oleum terebinthinæ, sabinæ, juneperi, macidis ; officinal ether sufficient iodine to leave some of it behind (6.) Brisk but less quick and violent fulmination, with undissolved. For the same quantity of essential oils as principally yellowish red vapours. Oleum neroli, in the above-mentioned tests, three drops of the ethereal bergami, limonis, aurantii, lavandulæ, spicæ, origani, tincture were added, in a place protected against draughts vulganis, petroselini, herbæ, copaivæ. of air. The drops of this ethereal solution were larger II. Quiet and noiseless evolution of yellowish red or than was anticipated; but I am not prepared to say gray vapours, accompanied with a rise of temperature. whether their size is to be ascribed to the presence of (a.) Many yellowish red vapours, considerable rise of iodine, or chiefly to the peculiar form of the lip of the temperature. Oleum cardamomi, melissä, majoranæ, vial from which they were dropped.

asari Europe; When testing with the ethereal solution of bromine, (6.) Few yellowish red vapours, with perceptible heat. a peculiar phenomenon had been observed, consisting of Oleum rosmarini, serpylli, hyssopi, anisi vulgaris ; a spreading out of the mixture, up the sides of the (c.) Few yellowish red vapours, little heat. Oleum watch crystals. Tbis was referred to the evaporation of thymi vulgaris, salviæ, millefolii, cubebæ, cejeputi, the ethereal liquid, and no further notice was taken of menthæ crispæ, matricariæ, arnicæ flor, anethi, foeniculi, it; but while experimenting with the ethereal tincture anisi stellati, carui ; of iodine, and noticing the same behaviour, there was a (d.) Few greyish yellow vapours, little heat. Oleum marked difference observed in this “spreading " from calami, valerianæ ; the presence of some oils, and attention was directed to (e.) Few gray vapours, little heat. Oleum nigellæ, it. The “spreading " consists in the uniform working cumini. up on the side of the crystal, towards the circumference III. Solution without vapours, but with a rise of of a larger or smaller quantity of the mixture, with a temperature. flapping or wavy motion, in some instances up to the 1 (a.) Considerable heat. Oleum cinnamomi Ceylon ; very edge of the vessel ; and then returning to the | 16.) Little and very little heat. Oleum cascarillæ, bottom again by forming streams from the upper margin cydoniæ, absinthii, cinnamomi Chin, caryopylli. down. Some oils show but little of this spreading IV. Solution without vapours and heat; motion, but mix quietly with the ethereal liquid, being (a.) Forming a homogeneous solution. Oleum cynæ, acted on by the iodine and exhaling the ether ; others, tanaceti, menthæ piper, origani cretici, sassafras, rutæ, again, while they are miscible with little disturb arnicæ radicis, petroselini seminis, sinapis; ance, subsequently commence to spread, gradually (6.) Forming two strata. Oleum asphalti, ceræ, running over the edge of the vessel, leaving but little succini. oil behind. Sometimes the commotion of the liquids is V. Partial and very sparing solution without reaction. such as to resemble a brisk effervescence or the pheno-Oleum amygdalæ amræ, rosa, petræ. menon of boiling.

The description of the colour as it is affected under While I at first supposed it to be all owing to the the influence of iodine, is a matter of some difficulty ; it evaporation of ether, I am now inclined to think that occurs very often that in the first or a subsequent stage of the composition of the various oils exerts its influence the reaction, a colour is produced which strongly reminds on this activity, while a neutral behaviour in this respect one of the colour of an iodine solution, but at the same of other oils can scarcely be founded on any grounds time so different as to be easily noticed. From the except peculiarity in their composition. I have in this similarity of appearance it was suggested to compare connection to draw attention to a phenomenon, which, them with the colour of a solution of iodine, and it seems to me, may be referred to the same reason. I wherever in the following observations, the expression refer to the peculiar motion imparted by certain volatile“ iodine colour” has been used, it is understood to apply oils to a solution of bichromate of potassa mixed with to a colour resembling that of tincture of iodine, diluted sulphuric acid, which was described by Dr. J. T. with alcohol to about four times its volume. From this Plummer, of Richmond, Ind., in the American Journal explanation, other expressions, such as pale, deep, of Pharmacy, vol. xxviii., p. 197.

| yellowish, reddish, &c., iodine colour, will be readily I regret that my attention had not been drawn to the understood. difference in this behaviour at an earlier season, so as to I have to remark yet, that the oils which I have used allow me sufficient time to report all the experiments in these experiments, were mostly obtained from reliable with the ethereal solutions, with a view of observing sources; in some instances the commercial articles were the particularities of the various oils in this respect. My used, in which case I took pains to examine them first in

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other ways, so as to be assured of their freedom from a while are miscible to a half syrupy greenish redadulterations. The physical properties of each of the brown liquid; the odour is scarcely modified. oils examined by me will be briefly stated.

Ether sol. iodine.-Some effervescence and spreading; In proceeding to state the reactions as they belong to the oil assumes a greenish yellow colour, while an iodine the various oils, I thought proper to give also the coloured resin-like mass collects on the bottom, which observations of Zeller, which in all cases will be dis- after mixing separates again; the oil has now a light tinguished by an affixed Z.

iodine colour, afterwards a brownish olive-green.

Ether sol. bromine mixes with the oil with a radiating REACTIONS OF THE VOLATILE OILS WITH BROMINE motion ; the colour is brownish yellow, with streaks of AND IODINE.

brown, which turn to a brownish purple, afterwards I. The Carbohydrogenso-Oleum Copaibæ. Thin, purplish black ; the supernatant oil is now olive green. colourless.

Bromine produces a very violent detonation and many Iodine.-Faint fulmination, with yellowish red vapours gray vapours; the residue by one drop of bromine is and considerable heat, developed on stirring. After the thin, pale yellow; by two drops, oily, dark olive green ; reaction the residue consists of a reddish brown iodine by four drops, thick syrupy reddish brown; the odour of compound, adhering like resin to the glass, and a syrupy the first is nearly unaltered, of the others more modified, liquid of a brown colour, changing into greenish by | but plainly like juniper. exposure to the air. Z.

(To be continued.) The reaction is not violent; the sediment has a blackish brown colour with a margin of yellowish brown, not iodine colour, the oil is yellowish, the whole miscible

PHYSICAL SCIENCE. without separating.

Ether sol. iodine.-Some spreading, mixes with little sediment and a brown yellow colour; after six hours,

On Platinum Standard Kilogrammes. greenish brown, little thickened, scarcely any sediment. | At a late meeting of the French Académie des Sciences,

Ether sol. bromine.—The reaction is accompanied by M. Regnault presented a copy of the report on the comwhite vapours and a green colour; after the reaction parison made in Paris in 1859 and 1860 of numerous the sediment is brown, the oil brownish green.

kilogrammes in platinum and brass with the platinum Oleum Cubebæ. — Oily consistence, limpid, a faint

standard kilogramme in the Imperial Archives. This greenish tinge.

report is printed at Berlin :Iodine.-Little heat, radiating motion, few gray and

i The Prussian Government possessed a platinum reddish vapours; at first the oil is violet; after mixing,

kilogramme, which on the 24th of October, 1817, was the colour is changed to yellowish brown, and the

compared by Arago and H. de Humboldt with the residue has the consistence of honey, and a somewhat

standard platinum kilogramme in the Archives of Paris. mod ified odour. Z.

| The Austrian Government, on its part, had a platinum Brisk reaction, without fulmination ; yellow vapours:

kilogramme made at Paris, which on August 20, 1857, the colour is blue, afterwards bluish green; after the

was compared by MM. Silbermann and Froment, in the reaction the sediment has a nearly black colour, with a

presence of M. Tresca, Sub-Director of the Conservatoire bluish green margin, around which the oil is of a lemon

İmpérial des Arts et Métiers with the standard platinum colour, growing fainter towards the edge. The whole is

kilogramme in the Archives. A comparison instituted miscible to a dark greenish liquid, from which the sedi

between the two platinum kilogrammes of Berlin and ment subsides again, leaving the oil of a greenish lemon

Vienna showed that they differed considerably. One of colour.

them at least must have been faulty. The only way to Ether sol. iodine.-Little spreading. The iodine

elucidate the question was to again compare these kilocolour quickly changes to yellowish and greenish brown,

grammes with the platinum kilogramme in the Imperial then greenish black. After six hours the oil is con

Archives of Paris. At the request of the Prussian siderably thicker and of a black colour, which in very

Government, the Minister of Public Instruction named thin layers appears blackish green.

la Commission, composed of MM. Regnault, Member and Ether sol. bromine produces white vapours and a

President of the Institute; Le Verrier, Member of the violet colour, growing deeper in a short time; after the

Institute and Director of the Imperial Observatory; reaction the sediment is of a deep violet, almost black;

Morin, Member of the Institute and Director of the the supernatant oil of a dark greenish blue.

Conservatoire des Arts et Métiers. On the other hand,

the Prussian Minister of Commerce and of Public Oleum juniperi bacca.—Thin, limpid.

Works named M. Brix, Conseiller Intime and Director Iodine.—The oil fulminates quickly, with evolution of of the Central Commission of Weights and Measures of violet vapours and much heat, particularly on stirring. Berlin, to co-operate with the French Commissioners in The residue is an oleoresinous, blackish brown mass, and again comparing the Prussian kilogramme of 1817 with some scarcely coloured oil, the whole miscible with some the standard platinum of the Imperial Archives. M. difficulty to a greenish brown, afterwards olive green Le Verrier was prevented by his occupations from perliquid. The oil from unripe berries fulminates more forming his share of the work. The direction of the violently and gives out with the iodine vapours, also experiments rested then with MM. Regnault, Morin, many of a gray colour. With old oil the reaction is of and Brix. But we ought to mention the active and shorter duration and shows less heat, the residue is intelligent aid given by M. Silbermann, M. Tyarn, and easily miscible, and has then a reddish yellow-brown by M. Delegil, philosophical instrument maker. If the colour, and the consistence of an extract. In all cases Commission had confined itself to re-comparing the the residve has a balsamic little modified odour. Z. Berlin kilogramme with the standard one of the Archives,

By the reaction much heat is produced and many and making the necessary correction, its work would gray, but few violet vapours; the residue consists of a have been less long; but on this occasion the Commission dark brown resin and an olive green liquid, which after proposed to itself a longer and more difficult problem:

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It proposed to study carefully all the circumstances which might influence determinations of this kind, and

PROCEEDINGS OF SOCIETIES. to find out all the circumstances which could occasion the errors found in the old stamped kilogrammes. Finally, ROYAL INSTITUTION OF GREAT BRITAIN. it was proposed to ascertain whether certain phenomena revealed by the recent progress of science, and which

A Course of Six Lectures on Light' (adapted to a Juvenile were formerly unknown, hare not vitiated the ancient

Auditory), by John TYNDALL, Esq., F.R.S., Professor of determinations, and obliged us now to modify or to make

Natural Philosophy in the Royal Institution. more precise the first definitions.

LECTURE IV. • (Jan. 2, 1862.) We will pass on to the definitive conclusion of the

NOTES TO THE LECTURE :report;

The white light of the sun is made up of an infinite number of rays To new

of different refrangibilities-Each particular refrangibility corresponds

to a particular colour; hence the number of colours involved in solar platinum kilogrammes verified at Paris with the platinum

light is infinite-But for convenience sake we divide these colours into seven, which are called primary colours. These are red, orange, yellow, green, blue, indigo, violet-Of these colours the red is the least refran

gible, and the violet the most refrangible; the other colours being vacuum. We have before shown the exactness of the intermediate between these two-The solar beam is resolved into these corrections we made for the displaced air. Thus, for colours by passing it through a prism: the coloured image thus formed

is called the solar spectrum-The colours of the spectrum, when suitably the Berlin platinum kilogramme No. 1 we have obtained

blended, produce white light-A colourless image of the coal points o the following results :

the electric light may be built up from the colours of its spectrum.

Some substances have the power of drawing the colours more widely From the weighing of 1859. Kl: = Ka+ 0.048 mgr.

apart than others; glass, for example, does this more effectually than

water, and bisulphide of carbon more effectually than glass-The » 1860. K6= Ka-0364 ,

drawing asunder of the colours by a prism is called dispersion ; thus,

the greater the distance between the red and violet ends of the spectrum, Of which the ...... K'b = Ka–0'158 mgr.

the greater is the dispersion.

When sunlight falls upon a body, a portion of white light is reflected

from the surface of the body-A second portion is reflected after it has "For the Berlin platinum kilogramme, No. 2, lately

entered the body to a greater or less depth-It is this latter portion made for the Prussian Government by M. Froment, which gives the body its colour-Different bodies have the power of we have found :

absorbing, or quenching within them, different kinds of light-A red body is red because it has the power of quenching all rays except

those which compose its red. K” = Ka

A blue body is blue because it has the mgr. 85.

power of quenching all rays except those which compose its blue“We can confidently state, then, that in a racuum

Brilliant crimson feathers, for example, if illuminated by pure blue

light, are as black as those of a raven ;-conversely, a pure blue would the Berlin platinum kilogramme No. 1 is too light by be black if illuminated by red light-When the human face is illumi. 0*16 mgr.; and the Berlin platinum kilogramme No. 2 is nated by a flame which contains no red rays, the lips and cheeks lose

all their redness ; in the same light red and crimson flowers lose their too light by 1.85 mgr.”

bloom-In fact, the colours of bodies aro entirely due to the light

which falls unon them. If the white light of the sun were simple inWe will now enumerate some of the most important

stead of compound, we should have only light and shado in the world: results obtained by the Commission. The platinum kilo but we should have no colour.

A metal heated to whiteness gives a continuous spectrum as long as gramme of Berlin verified by Arago and de Humboldt

the metal remains in the solid or liquid condition-But when a metal in 1817 was too light by 12.020 millimètres. The brass

has been reduced to vapour, and when that vapour is rendered lumi. kilogramme of the College of France, too heavy by nous by intense heat, the spectrum of the vapour is usually composed

of brilliant bands-Every metal has its own distinct system of bands3.846 millimètres, has not sensibly changed in weight

When metals are mixed together so as to form alloys, the bands of during ten years' exposure to the air without covering each metal are produced in the spectrum of the alloy -Tbus, knowing

the bands that each separate metal produces, we can determine from in the glass case of a balance. A similar platinum kilo

the spectrum of an alloy the metals of which it is composed-The gramme, after having been several times in vacuo, where

bands of the metal also exhibit themselves when the salts of the metal it often remained for several days, did not show the are raised to a sufficiently intense temperature.

A luminous vapour absorbs those rays which it can itself emit slightest alteration in the weighings, nor when weighed

Thus the light of incandescent sodium vapour is intensely yellow ; but in the air. When a platinum kilogramme is placed in when a beam from the electric lamp is sent through this vapour, the

yellow rays of the electric light are intercepted--The sun is supposed each pan of a balance, and successively weighed in air

to be composed of a solid or liquid central portion, which of itself more and more rarified, the changes which take place in

would give a continuous spectrum-But this nucleus is surrounded by the apparent weight correspond exactly with the weight a flaming atmosphere, through which the rays from the nucleus have

to pass-This solar atmosphere, or pbotosphere, as it is oiten called, of air displaeed in each case. In other words, an

intercepts those rays of the nucleus which it can itself emit, and hence abnormal condensation of the air on the surface of the the solar spectrum is alw

the solar spectrum is always furrowed by dark lines (Fraunhofer's

lines) From these lines we can determine the metals which produce two platinum kilogrammes does not produce a sensible

them: and in this way it his been found that many of the terrestrial change in their relative weights. If there exist a con- metals are present in the sun. densation of this nature, it produces the same effect on each kilogramme. The condensation of air on the sur- With regard to the duration of the impression upon the face of a platinum kilogramme is in no case sufficient to retina, there is a little experiment which all my pupils alter sensibly the apparent weight. Two brass kilo present can make for themselves, and which is a very pretty grammes have the same relation of weight, whether they | one. It consists simply in taking a knitting needle, sticking are weighed in vacuo or in the air, and the apparent a little silvered bead on the top of it by means of marine weight is corrected by the weight of air displaced, calcu- glue or sealing wax, and fastening the other end of it firm, lated by the ordinary principles of physics. Under the and then striking it so as to cause it to vibrate. When conditions in which the Commission operated, glass pro- you allow the sunlight, or even the light of a lamp or candle.

on to fall upon the bead, you see upon striking it, the needle duced no abnormal condensation of air nor humidity on to,

vibrates, and the bead performs certain excursions, and its surface. The surfaces of platinum plates did not

goes on describing the most beautiful figures. condense a quantity of hydrogen appreciable by the

This I say is an experiment that each can make for him. most sensitive balance; or, not to go farther than is

self, it is extremely pretty and well worthy of your attenwarranted by experience, the condensation, if it exist, is exactly the same under a pressure of 6.4 millimètres as under the ordinary pressure of the atmosphere.

1 Reported verbatim by special permission.

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Now, let me show you what may be done in this way. illuminated will appear perceptibly thicker than the portion We have here such a piece of vibrating rod, it is not not illuminated. This explains the effect I have spoken exactly a knitting needle, but something stronger and better made. I will illuminate this bead by a beam from the electric lamp and project an image of it upon the screen. You see the little spot of light upon the screen ; I will now bring that quite to a point, and if I cause this to vibrate, you will see what beautiful figures I get. Here you see we get a curious figure of 8, which denotes the kind of vibration of which this rod is capable. You can also get figures far more beautiful than that. If I touch it with the fiddle-bow, I shall produce a figure of 8 with a fine crimped outline. Now instead of this vibrating bead I will take simply a knitting needle, and introduce it into the beam of light as before. We will alter our lens so as to bring it in focus to a little point, and cause it to shine brightly upon the screen. Then I touch that, and you have a beautiful circle of light. See what a lovely figure you have when I touch this knitting needle with the fiddle-bow. These experiments, and numberless others, you can all make for yourselves, and they are really well worthy of your attention.

Now, a number of interesting and ingenious toys depend for their beauty upon this principle of the persistence of image upon the retina of the eye ; and among the number is the chromatrope. I will just show you one that will of, viz. that the farther you go away from the object suffice. Here we have a specimen, and we will throw our the greater is the amount of irradiation. Hence it is that, fine disc of almost solar light in intensity, upon the screen, looking at the new moon, which is a body 240,000 miles and then bring the chromotrope to a perfect focus, so that from the earth, you see it encircles the dusky portion of the definition may be perfectly sharp. If I now turn the the moon and appears to belong to a larger sphere altogether. winch, you see the curious appearance of motion produced, This effect can be illustrated in another way. You see and if I turn it the other way the motion you see appears this fine platinum wire stretched from one end of this to take place in the other direction—all these more-instrument the other. When I connect it with the galvanic ments depending for their effect upon the power of the battery, the mysterious power which rushes through it eye to retain an impression for a certain definite period of from the battery down stairs will heat the wire to a very time.

high degree of incandescence, and make it white with I want next to draw your attention to another subject heat ; and then, I think, those at a distance will observe mentioned in our last programme (for I wish to make clean that the wire appears to augment in thickness, that is, its work of our programmes, and not desert the memoranda apparent thickness will be much greater than at present. until we completely finish them). I mentioned there a The wire being now heated, appears to me to be much peculiar effect upon the eye which I have called irradiation. thicker than it was formerly, and I dare say to those at a It is a learned word, but it is very well that you should distance the difference will be more apparent. If I brighten understand such, because you will frequently meet with that wire, you will find it will appear still thicker. I can these terms when you read, as I trust you will read, books do so by sending a more powerful current through the wire. on Natural Philosophy. You know that when I look at This I shall be enabled to do by shortening the wire, and any of you, as we explained in our last lecture, there is thus lessening the resistance which it opposes to the pasan image painted on the back of my eye. The friend sage of the current. You see how intensely it is illuwhom I see before me is, at the present time, depicted minated now, and I have no doubt it appears thicker with perfect distinctness at the back of my eye, turned, than in the former experiment. I think the wire will bear with his head downwards. At the present time he is yet a little more heating. I will shorten it a little more, moderately illuminated ; but suppose, instead of the mild and then I will diminish the light of the wire by interlight upon a little boy's face illuminated by the lamps posing a coloured glass. [In this experiment the heat was that light this theatre-suppose him intensely illuminated so intense that the wire fused and parted.] The heat has -suppose his face to shine with the brilliancy of the sun, fused my wire, but I will try again. I want to show you or very nearly so, then, in virtue of the intensity of the the effect of darkening by cutting off a portion of the light light, the image of his face on my retina would appear a of this wire. little too large and would encroach upon the circumjacent You will have the wire bright as before until I interpose space, and I should see his face larger than it really is. this piece of coloured glass. Look through the glass and I should also see it so much the larger the farther I went at the wire, and compare that portion of the wire which away from it. I have here drawn two rings, and I would has the light cut off with the rest, and you will see the ask you to direct your attention at the present time to them. difference. If I were to take a still darker piece of glass, The black line is exactly the same thickness as the white the wire would appear much thinner, exactly as the size one. I think those at the distance if they were asked of the moon, looked at through a dark glass, on the Alps, which of these lines were the thickest, even with the present becomes apparently less. illumination, would be inclined to give the preference to So much, then, for this question of irradiation. Now the white lines. But, in reality, the one is not a bit thicker we come to the main subject of the day's lecture, and I than the other. I will ask Mr. Anderson to hold the dia- will go on building up my argument by facts. I will program in the light, and I think you will see that the white ject, first of all, a slice of light from the lamp, upon the line is apparently the thicker of the two. Those at a screen, by means of this lens. Now, would you suspect distance will see the effect of this more clearly than those that there was within that image-white, and beautiful near at hand; and if Mr. Anderson now partly withdraws and colourless as you see it there would you suspect that, that white ring from the light and lets one side be illumi- in that white image, you would have the most splendid nated and the other not, I think you will be able to perceive red, the most vivid orange, the most burning yellow, I the difference in the thickness of the ring itself; the portion I was going to say,- and green, blue, indigo, and violet,

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all mixed up in that space of white. So that that light the colours are thrown simultaneously into the eye, and, consists, not of a simple thing, but of the blending of all being blended there, produce the impression of white light. these colours-colours so intense, so beautiful, that no Now this must satisfy you that the white light that painter could imitate them.

comes to us from the electric light, and also from the sun, I will now take a little prism of glass and refract this is composed of rays of a variety of colours. Some of beam of light upwards; you now see the image is refracted, these rays are more capable of beirg bent than others, and and goes up as you remember it did in our former experi- that is the reason why we are able to separate them. The ments. But it you look closely at that image you will blue is more so than the red, and the consequence is that, notice a little colour-a little red at the bottom, and when we send them both through the prism, the one is blue at the top. Now, I will take a larger and more separated from the other. powerful prison, and will cause the light to pass through I will now make an experiment with a prism of another it, so that it will be refracter upwards, and I think this kind. We have here a hollow glass prism with the sides powerful prism will be sufficient to cast the beam high up cut and polished, and in this vessol is placed the liquid, upon the wall, and you will then find this white light bisulphide of carbon, that you see mentioned in the reduced to its coloured components. This is the grand memoranda of this day's Lecture. This gives me a discovery of the great Newton-for it was Sir Isaac more richly-coloured spectrum than the glass. Now I Newton wo discovered that the white light that you see will place this bisulphide of carbon prism on the stand. is produced by the blending together of these splendid The light will come ihrough this lens, and strike upon the and beautiful colours.

surface of the prism; and it will pull the blue aside more I want now to carry you through the proofs and argu-than the red, and thus will separate the colours better. ments and considerations which depend upon this point. Here, you see, the colours of the spectrum are richer First, I will try and produce the self-same coloured image, than those produced in my last experiment. or spectrum as it is called, by deflecting a ray of light Now, bear in mind that the beam of light, before sideways instead of upwards. I will place my slit verti- refraction, would go on in a perfectly straight line. It is cally, thus, and if I make my coal points touch, a beam of pulled aside by the prism, and you see the blue is pulled light will pass through. I interpose my lens, and place more aside than the red, and is, therefore, said to be the the prisin behind the iens and turn it so that the beam of more refrangible colour, and the red is the less refrangible light shall strike the prism. There, you have this beauti-colour. Newton divided this bluish portion of the ful image upon the screen-perfectly sharp and brilliant. spectrum into three kinds; he called the first, that is the I will now see whether I cannot actually squeeze those portion next the green, blue; the next indigo, and the colours together, and mix them up so as to destroy that extremity of the spectrum he called violet. Thus we coloured image that you see. I place a lens behind the have the seven primary colours : red, orange, yellow, prism and I will see whether or not by means of this lens green, blue, indigo and violet. I want now still more to I can actually recompose the light-reblend it-remix it augment the dispersion--remember that word; it means so as to produce the original white image. There, you see the pulling aside the colours of the beam. The amount the light is so reblended that you have the various colours of dispersion is expressed by the length, from the in a great measure destroyed. Supposing, now, I cut off extreme red to the extreme violet : the greater its length, with a screen the red which is on this side, what do I leave the greater is said to be the dispersion. Having now You see the blue coming in on the other side; and so you see sent a beam through one prism, I will catch that coloured when I take a prism of this kind I can actually separate one beam as it issues from this prism, and send it through image from the other. Now, I will try to deflect the red another, and thus I can pull it round still further. In beam away. I can do it by partly interposing this small that way I expect to get a spectrum that will stretch prism which you have had already. Here, then, you see almost across the entire screen. I can twist that beam the red is pulled away and now stands beside the blue; round so as to bring it upon the screen with a greatly and again, if I deflect the blue and throw it to the other augmented dispersion. There you see you have this side, you see the red start out. These two colours which glorious exhibition of colour actually stretched almost I thus separate when blended together produce white light. across the entire screen. Now you would call no portion They are called the complementary colours.

of this image white, the paper has actually become Now, let me see whether it is not possible, from this coloured in virtue of the light falling upon it--the light coloured image, to actually take the light of which it is has painted it. In point of fact all the colours that we composed, and to rebuild from it the very coal points from see in Nature are produced in this way. Colours are which the light issues. Let me see whether I cannot not inherent in the bodies themselves at all. It is do that. These rays of light are, so to speak, the rough the light which falls upon them- which is in fact the bricks and mortar of which the coal points are built. paint-which gives beauty to every colour. However, I Here, then we have the light which composed the image, shall derelope this subject more clearly as we go along. and now we have recomposed the coal points from which Now here at this end you see we have an intense beim of it issued.

red light. I will let it fall upon a red object, and you Let me now make an experiment of Newton's, for you will see that the object will appear red in it. I will show to see the way in which he satisfied himself that the you this bunch of artificial flowers which are red, with blending of these colours produced white light. He took green leaves : when I allow the red light to fall a circular piece of paper, or paste-board, and he divided upon these flowers I think you will see that they are this circle into various parts, and coloured those various red, and you saw that they were red when the white parts with the colours of the spectrum-one part red, light of the gas fell upor. them. Now this is a another blue, another orange, and so forth, and then he very important point which I wish you to perfectly set in it motion. He knew very well that by setting it in understand. Why are these things red when the white motion, because every colour remained on the retina light of the gas falls upon them? Simply because the during the time of the revolution of the circle, he cloth, or whatever it may be, or the colouring matter used actually threw all the colours together into the eye, and in its manufacture-the dye of the cloth-has the power re-blended them. And in that way, when he set ihe of completely quenching, absorbing, drinking in, destrorcard in motion, he produced a white disc. Now I have ing the blue, green, and yellow-all these are destroyed, here such a circle, not as Newton made it, but of glass and the only light which it is capable of sending back is painted with transparent colours, and I project the image the red. If what I say be true, if I pass these flowers of this upon the screen, when you see that by turning into another coloured light, you will find that it has no his handle the colours completely vanish, because all power of showing itself red, or any other colour. If I put

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