« AnteriorContinua »
use of two I,imps, and let light from both fall upon an object such as this ball (b) which I have suspended here. It will show you the difference between a perfect shadow and an imperfect one, or ha'f shadow, very clearly. I have now divided into two equal parts the battery which I have hitherto made use of. In the experiments that we have hitherto made we have used sixty cells. I have now separated this it to two batteries, each possessing thirty cells, and each of these batteries I have connected with one of these electric lamps. Now you will see the principle which I am endeavouring to make clear to you when I cause these two lights to operate. Here fat I) we have one light which will strike that ball and cast a shadow of it upon the screen (at »). There is the shadow of the ball upon the screen ; and now I have another light (/') at some distance from the former, connected with its ow.i battery, which casts another shadow of the ball on the screen (an'). These two shadows overlap each other, so that you see there is a certain portion which is undoubtedly shadow to one lamp, but not shadow to the other. And here in the
centre (c) we have a region of perfect shadow. This (» and »') being the penumbra. On each side we have the penumbra, and if you suppose the light to come from a ring of lamps, this penumbra will extend all round, and we shall have a circular space shaded. This is exactly what takes place with the sun in regard to the shadows that the earth and moon cast on space. I have drawn them here, this («) being the earth, and this (s) the sun. Owing to the greater magnitude of the sun, the region of real shadow is represented by this dark cone. It is, as I
shadows, which are consequent on the passage of light in straight lines through space.
I pass now to another very important portion of our subject, and that is to the subject of reflection, and I want to make this Bubject of reflection perfectly clear to everybody present. Philosophers, the wisest and the greatest, when they think, and when they reason upon these things that we call light and magnetism and electricity, are actually forced to imagine these things to be like something tangible. Mr. Faraday, I have not a doubt of it, imagines that magnetism is something moving about in lines—straight lines and curved lines. He imagines this, but it is all imagination, for human eye has never seen it. And so with regard to light; we are obliged with the eye of our mind, wi'.h the eye of our souls, so to speak, to imagine that we see these thing, and to look at those things which are imperceptible to the eyes ef our bodies. Now, it is of infinite importance, I think, that boys particularly, and if you will allow me to say so, girls too, should have clear and distinct ideas of how this thing that we call light is reflected. Here is a bagatelle board, known to all of you; if I take one of these balls and drive it plumb against the other side, what occurs? It rebounds. That is a case of reflection, true reflection, so that you see when I strike this perpendicularly,—to use a very learned word which of course all of you will knowby-and-by,—when I strike this straight against the side, the ball rebounds along the same line, it returns along the line on which it struck the surface. Light does exactly the same. If a beam of light strikes plumb against the surface of a plane mirror, it rebounds; it is reflected back exactly as that ball is reflected back; 1 and suppose a billiard player wishes the ball to rebound in a sloping direction, after striking the side of the billiard board, what does he do r He does not strike it plumb; no, he wisely strikes so that the ball shall hit the board obliquely; and now when I do so, if you observe the path of the ball, you see it will strike here, and be reflected back on a line obliquely to the surface against which it strikes. Now, what I want to imprint upon your minds, and never to forget is this :—Supposing you have a line (o b) drawn perfectly plumb and perpendicular to another one (cd), and supposing here to be a ball (<)> and that I drive that ball right against that point (a) where this line is drawn; if that ball were perfectly elastic, and if the surface against which it strikes were perfectly elastic, then it would rebound along this line (a/); it would be exactly
Section across m n. have said, a convergent cone. It comes to a point, and all round that region of perfect shadow you have the penumbra, the section being as seen across m n. I cannot here enter into all the minutiai of these things, but I have written down in the notes sufficient to engage your attention, and I trust you will take each statement that I have made, dwell upon it when you go home, and make yourselves perfectly clear abont it. What I .want you to remember is this,—that when the luminous body is less than the illuminated body, you have a divergent cone, surrounded by a penumbra, and when you have a luminous body, such as the sun, larger than the body illuminated,— larger than the opaque one,—larger than the body which casts the shadow—then 1 trust you will make it perfectly clear to your minds that you must have a shadow of the ahape of a convergent cone surrounded by a penumbra; and astronomers sometimes see these vast conical shadows moving through space during eclipses. So much for
equally inclined on each side to this line (o b), and would fall »s much upon one side of the perpendicular as it did upon the other; the two angles \g and A) being exactly equal to one another. Now, I hope you see this clearly; it is very gross and very simple, but it is absolutely essential to the understanding of the law with regard to light, for light does just the same. Light, when it strikes obliquely against a reflecting surface, is driven back; so that the reflected ray, as it is called, makes the same angle with the perpendicular as the direct ray. To make this plainer, I will try, in the first place, to show you the general fact of the reflection of light. I will take, in order to show you the effect of this reflection, a beam from a lamp, as I have already done. Now, how is this reflection accomplished? You see this beam passing straight from the lamp. Here I have a piece of silvered looking-glass; I throw that into the path of the beam; and look what takes place. There you have that splendid cylinder of light passing through the room. You see as I twist the
mirror the reflected beam passes over the gallery, and thus it is evident I can make the beam to course along and be reflected as I please. The great Sir Isaac Newton imai:ined that light was due to small particles projected away from the luminous body with intense Telocity, and that these particles were reflected in precisely the same manner as we have seen the billiard ball to be reflected. I should not like you to dwell upon this as a truth at the present time. We nave reason to believe that this is not the case, but it is too early to think about that at present. Now, I want to examine this matter of reflection more fully, and before I do so, let me just draw your attention to that splendid beam of light. The slightest motion of the glass causes it to m*Ve through a very vast space. All these things ought to be taken, and are taken, advantage of by philosophers. You saw that beam of light moving about. It has no weight. If you wish to enable yourselves to see the motions of the wheels of your clocks and your watches you put hands to them and make them traverse a large circle, in order to be able to divide that circle into hours and minutes, and you can see the hands travelling oyer those hours, over those minutes, and the larger your circle the more minutely can you subdivide it. There are friends of my own here who know how very necessary it is in the large instruments—large theodolites—to have a large circle in order that that circle may be divided into very small parts; bnt weighty indipes like these hands would be utterly useless in any scientific experiments, and philosophers, therefore, make use of an index without weight—a glorious beam, such as you saw there going over the room. They attach a bit of looking glass to their little magnets, and the slightest motion of the magnet pauses this long index—and you can make it as long as you liker—to move through a large space, and in this way we employ an index of light which possesses no weight. Now I want to develope further, if I can, this doctrine of the reflection of light; and in order to make you understand it thoroughly and perfectly, 1 have placed this lamp (!) here, and have placed a piece of common looking-glass (go) in front of the lamp. Now what am I doing here?
You see the face of that looking-glass (g a), it is exactly at right angles to the light. This beam (2) is perfectly plumb, is perfectly perpendicular to the surface of this looking-glass. Very well now, but you. see that in this case, when the beam strikes against the surface of the looking-glass, it is reflected directly back again and strikes the lamp, and a portion of it is reflected back upon the audience in the line in which it fell upon the looking-glass. Now I have here drawn the sweep of a large circle; this arc J have divided into exactly twenty' equal parts. You see the index (a 4) that I have here. When I turn it round it causes the looking-glass to move. This index is perfectly perpendicular to the surface of the looking-glass. Wei), 1 will pjflce it here at Now you remember what I'have said about
the billiard ball; if what I have said be true, inasmuch at that ind,'x (a b') is plumb to the surface of the mirror (</' a), and inasmuch as the direct ray of light will, as I have sai'l, fall as much on this side (c) of the index as the reflected ray falls upon the other side (4), then the ray of light will strike the youth who is in the direction of this line (a c). The apparatus is placed with perfect accuracy, so that you see this direct ray of light (a I), and that reflected ray of light (a c) make equal angles with this perpendicular (o J').
Now, I have endeavoured to define a term which is used incessantly in writings on this subject, and which must be perfectly remembered. The angle you see here (I a b') between the direct ray and this index, is called the angle of incidence; and this angle between the reflected ray and the perpendicular (4' a c), is called the angle of reflection. I am satisfied every boy could express this law. Now I will ask you a question,—Is the angle of incidence unequal to the angle of reflection? Is it, or is it not? [Voices: "No, it is not."] No, it is not; the angle of incidence is equal to the angle of reflection; and thus we have arrived at the law, simple as it appears, and simply as it is proved, which lies at the foundation of optical science. Another point I want to direct your attention to is this (and remember what I have just established—that the angle of incidence is equal to the angle of reflection): I bring the index back to mv former position [a 4), and I will allow the beam to fall plumb upon that minor (ag). Here it is. You see the beam returns exactly along the same line, as I have already proved. Now watch me: I will cause the index to move | I will push it on. Now both the index and the reflected beam start from one and the same point; they are like two boys setting out to run a race, for they start fairly; but watch their progress, I push the index to No. 2. Where is the beam? The beam has gone farther; the beam is now at 4. I push the index to 4. Where is the beam now? It does not coincide with the index; the beam is now exactly at 8. I push the index to 6, and the beam is now at 12; I push it to 8, and the beam is now at 16, exactly; I push it to 10, and the beam is now at 20. What is this beautiful law ?—for it is a beautiful law, and a law of very frequent application. I have expressed it in my notes that the reflected ray turns round with twice the velocity of the mirror. You will find a learned way of expressing that in books where they state that the angular Telocity of a reflected ray is exactly double that of the reflecting mirror.
Many other things equally interesting flow from the same principle. I have met sometimes, in walking through Regent's Park, and I daresay many of my hearers have done the same, with an old man standing with an instrument mounted on three legs, like a photographer's camera, and he had, supported upon these legs, a system of tubes much more cunningly devised than this [exhibiting a square mahogany tube with four rectangular elbows], but still exactly the same in principle.
Well, this is supported upon the legs and he desires people to look through his tube; and he puts, cunningly enough, opaque things between, for he has these tubes prolonged a little so as to make it appear as if you looked stiaight through, as his object is to make you fancy that you actually look .straight through these tubes, and in consequence he prolongs this and leaves a space between. He puts his hand in and wants you to believe you can tee
through his hand. He puts a piece of board in there, and
I have now to beg your attention to another effect of reflection, and it is very instructive indeed. I have here a pair of mirrors. If I place that candle between those two mirrors, set together at an angle, you see more than
one image of it, you see a series of images; and the larger
There, is now another point I think I ought to direct your attention to, and that is this, that when you look into a mirror vour face is laterally inverted; for instance, I believe that my hair is really parted to my left hand, but if I look here I see a gentleman in this looking-glass with his hair parted to the right hand, and thus we are laterally inverted when we look into a looking-glass; and photographers often have people complaining when they find themselves represented exactly as they are, because they quite forget that they do not know themselves really as they are; and when a photographer sets the parting of the hair on the proper side— the side which other people see, they are sometimes very angry with the photographer. They say he has misrepresented them, forgetting, in fact, that they have been using an instrument all their lives which inverts them
in this lateral way. I have written a word here backwards, but if I look at it in this looking-glass it is perfectly legible, as you will see it to be when I cast it on this screen. Thus when compositors set up their type, it has to be printed upon a sheet, and therefore the type is always the reverse of the printed sheet. Here I will cause this backward word to be reflected upon the Bcreen, and you will then see what it is. You have it now plain enough, D—O—G dog, it being inverted by the reflection.
I have now to pass on from inversion by plane surfaces —by such things as looking-glasses—to reflection by curved surfaces. We have here a splendid mirror; here is one less splendid, but equally good. I do not know how you could leam any philosophical matter without practical instructions. The most learned man may discourse upon natural philosophy, but he tan never get the thing into his hearers; you can never learn natural philosophy by hearing it, or by reading about it. Reeding and hearing it are both very good; but let each boy present, when he eoes home, who feels an ambition to become a natural philosopher,— and indeed it is a glorious vocation,— try to repeat the experiments for himself. Let him, if he has sufficient pocket-money, buy » little convex mirror, or a little concave mirror, as I have said, and try to repeat the experiments for himself, and not give way if he finds it difficult. Let him persist, and repeat his experiments again and again, and with a kind of moral responsibility, and he will then, and in that way only, become a natural philosopher. That is the only way to master the truth of the thing.
Now, supposing I have a ray of light, and that ray of light falls at an angle upon a plane reflector, you know how it will be reflected, making the Fame angle to the perpendicular as did the direct ray. Here, again, let us have another piece of reflecting surface inclined to the first one in that direction; and let us suppose a ray of light parallel to the first ray to strike on this plumb, it will be reflected back again to where it started Irora. And now we have another piece of looking-glass inclined in the opposite direction to the first; and let us have a third ray of parallel light falling upon that; it will be reflected as the others have been, and thrown back. Thus you see that, by this arrangement of eloping the mirror, we shall get all the rays thrown back so as to cut one another in the samo point here. Now, if you had a great number of small bits of mirror, you might so arrange them that the reflection from each should come towards that same point, and in a concave mirror, as it is called, we do that. In a mirror of this sort, formed by the union of surfaces of that kind, all the rays that fall upon it on reflection are forced to come back and unite at a single point crossing each other. I will now make an experiment with this mirror so that you may see the track of the beam in the manner I have already indicated. [The diverging [rays from the electric lamp were then allowed to fall, at an angle on the concave mirror, which reflected them back again to a focus, the conical form of incident and reflected rays being clearly visible in hazy atmosphere.]
How splendidly these beams are reflected! How they are squeezed, and converged and brought together to a "focus," as we call it. There you see the focus ; how bright it is! And if that mirror were perfect, you would find that the carbon points of the lamp, that are intensely illuminated, would rebuild themselves up there, and you would have them here depicted in the focus with the utmost accuracy; even now I can sec a blurred image of the coal points on this screen when I interpose it, so that the rays of light that issue from that lamp are made use of by this mirror to build up the image again; they are the bricks, so to say, by which the coal points are here built up in the focus of the mirror, and as I cause the mirror to move—if I tip it up—look at that splendid cont of light produced by the convergence of the rays
after reflection. This then will illustrate the law of reflection from this concave mirror.
I have now a convex mirror, and when I set it down here for a moment we shall see the difference between this and a concave one. This convex mirror bulges out, you know, in the centre, while, the concave is hollow in the centre. Now, what takes place with a convex mirror is this. Supposing the rays of light fall upon it, they are reflected back again, but spreading out instead of being squeezed together to a point, and if you look at the mirror you will see the picture of the rays, as it were, going right through it and intersecting at a point behind the surface. With the concave mirror you remember that we had the rays converged and caused to cut in a point in space actually before the mirror, but here in this convex mirror you see there is no real cutting of the rays; they are all reflected and caused to diverge. The intersection behind the mirror is merely an appearance, and hence this cutting point, which is the focus of the mirror, is called an imaginary focus, in opposition to the real focus, which as I have said in the Notes, is actually formed in space in front of the concave mirror.
MANCHESTER LITERARY AND SCIENTIFIC SOCIETY. Ordinari/ Heeling, December 24, 1861. J. P. Joule, 1.1.. U., President, in the Chair. Mb. Buockbaxk exhibited some samples of steel manufactured by Mr. Bessemer's process. These specimens had been bent and twisted cold, and showed a remarkable degree of ductility. He stated that the Bessemer steel was one of the most plastic and manageable of metals— more so even than copper. It could be bent, flanged, or twisted, either hot or cold, without annealing, and over a considerable range of temperature—which is not the case with ordinary steel or copper.
A plate of 18 inches diameter had been forced through a series of dies until it formed a tube 13 feet long and if inches diameter, without any crack or flaw.
A ring of metal could, at one heat, be hammered into a die to form a locomotive chimney top.
In drilling a circular hole into a plate, continuous shavings ore formed—whereas, in copper, or Low Moor plates, or any other metal, the shavings break into pieces iV». long.
i Thin sheets of the Bessemer soft steel can be bent backwards and forwards hundreds of times without a fracture, and are almost as flexible as paper.
Meeting, December 16, 1861.
E. W. Binnst, F.R.S., F.G.S.,utthe Chair.
Dr. Edward Morgan was elected a member of the Section.
Dr. Wallioh kindly presented to the Section for mounting several specimens of material, from his private collection, containing Blddulphia of various kinds, and other diatomacece, from Guernsey, St. Helena, &c.
Mr. Thomas H. Nevill presented to the Section eight slides, mounted from the specimens of Soundings, No. 131, taken in Lat. 51° 48' N., Long. y° 8' W., off the south coast of Ireland, in forty fathoms, presented by Captain Moodie, of the R.M.S.S. Canada. Mr. Nevill reported that the specimen contained Entosolenia Marginata, EntoEolenia Squamosa, Lagena Vulgaris, Textularia, Rotolina, Miliolina. Numerous spines and plates of Echina; calcareous pnsms from shells, &c, &c, all water-worn. The sand is composed of about half calcareous and half siliceous material.
Mr. Latham proposed that the subject for discussion at
the next meeting should be "On the Cause of the Metallic Liustie on the Wings of the Lepidoptera, both Diurnal and Nocturnal," which was agreed to. Mr. Latham also reported upon the Ovum presented at the last meeting by Mr. Leigh. Mr. Latham presented to the Section a slide, mounted with a portion of the elytra of the Platyomus subcostatus, from Venezuela j also, an oak spangle with stellate hairs.
Mr. Binney exhibited mounted specimens of Fossil wood, from Standish, near Wigan; Trigonocarpon oliviforme, from the lower Lancashire coal bed; and the palate of the Psamrr.odus porosus, from the mountain limestone, County Armagh.
Mr. JoYpresentedmountedsectionsof coal from Bohemia, showing woody fibre.
Mr. WnAlley exhibited living ova of the Trout, one month old.
Mr. Brothers exhibited a section of Agate from Siberia; Stentor Miilleri, &c.
NOTICES OP PATENTS.
791. Preserving Fermented liquors. Henry Mbdlock, Great Marlborough Street, London. Dated March 30, 1861. This improvement consists in adding sulphurous acid or a soluble sulphite to beer and similar liquors after fermentation, or introducing these agents into the bottles, casks, or other vessels about to receive the same. Such addition has the effect of preventing acetous fermentation, or the tendency to become sour on keeping.
794. Lubricating Compound. O. Eaulb, Liverpool.
Dated March 30, 1861. For the preparation of a cheap and efficient lubricating agent, the patentee recommends the employment of an oil combined or incorporated with lime water.
The addition of lime water is not calculated to aid in an economical point of view j and wherever such a material is applicable it is more than probable that soap water would answer the purpose equally well.
Grants of Provisional Protection for Six Months.
2658. George Davies, Serle Street, Lincoln's Lin, London, " Improvements in lamps for burning coal oil and similar fluids."—A communication from Joseph Thomas, New York, and Joseph Tindalt Van Kirk, Philadelphia, TJ.S.—Petition recorded October 24, 1861.
2716. Eugene de Bassauo and Adolphe Brudenne, Brussels, "Improvements in the manufacture of stearine." —Petition recorded October 30, 1861.
2815. Frangois Henry Marie Come Damicns Chevalier Finis de Lacombe, Paris, "Improvements in generating hyiirogen gas for illuminating or other purposes, and in apparatus used therein."—Petition recorded November 9, 1861.
2861. Henry Bird, Liverpool, "Improvements in the construction of bottles and other vessels, and in stoppers for the same to indicate that they contain poison."— Petition recorded November 13, 1861.
2906. Simon Dede, Rue Duvivier, Paris, "A new process of discolouring, purifying, and improving varnish, oil, resin, gum, ether, wines, spirits and other matters through the application of compressed air."—Petition recorded November 19, 1861.
2924. George Henry Polyblank, GracecUurch Street, London, "A new or improved method of protecting and preserving photographic and other prints, water-colour drawings and other works of art from injury and decay."
2961. Alfred Vincent Newton, Chancery Lane, London, "An improved method of removing and preventing the
formation of calcareous and saline deposits in steam boilers."—A communication fromLewisBaird, Cambridge, Massachusetts, U.S.
2991. William Clark, Chancery Lane, London, "Improvements in the construction of parts of electric telegraph belt apparatus, and in apparatus used in making the same."—A communication frcm PierreDesirePrud'homme, Boulevart St. Martin, Paris.
2997. Henry Wilde, Manchester, "Improvements in magneto-electric telegraphs, and in apparatus connected therewith."
3002. Peter Spence, Newtown Heath, near Manchester. "Improvements in the treatment of ores for the manufac ture of sulphuric acid, and in apparatus connected there with, which apparatus is also applicable to the treatmenl of ores for separating metals therefrom."
3044. Richard Archibald Brooman, Fleet Street, London, "Improvements in albums or books for containing and showing photographic and other pictures, and in slides for the same."—A communication from Henry Strauss, Paris.
Inventions protected for Six Months by the Deposit of Complete Specifications.
3112. Marc Antoine Frangois Mentions, Furnival's Inn, London, "An improved means of defecating and purifying cane and other saccharine juices."—A communication from Louis M irie Amand Achille de Courson de la Villeneuve, Caen, Normandy, France. — Deposited and recorded December 12, 1861.
Notices to Proceed.
1958. Peter Spence, Newtown Heath, near Manchester, and James Mellor, Manchester, " Improvements in separaing copper from its ores."
2023. Richard Archibald Brooman, Fleet Street, London, "Improvements in coating wire with copper, silver, gold, or other metal or alloy, in order to prevent oxidation."—A communication from Martin Miller, Vienna.
2067. Richard Archibuld Brooman, Fleet Street, London, "An improvement in preserving meat and other animal substances."—A communication from Jean Pierre Lies-Bodard, Strasbourg, France,
2347. Rene Prudent Patrice Dagron, Paris, "An improved microscope to be used for exhibiting photographic views and productions."—Petition recurded September 19, 1861.
2441. Pierre Alexis Francisse Bobceuf, Paris, "The preparation and application of certain new hemostatic and antiseptic agents."—Petitions recorded September 30, 1861.
3005. Jules d'Adhemar de Labaume, Dorset Terrace, Clapham Road, Surrey, " Improvements in machinery for cooling and freezing water and other fluids."—A communication from Edouard lilee, Rue d'Amboise, Paris.— Petitions recorded November 28, 1861.
To the Editor of the Chemical News.
Sir,—In the Chemical News of the 7th December, you inserted a question that I wished to ask Mr. Newlands, to ascertain whether his new system of nomenclature would be of any service to chemists: my question was, " What name would he give to albumen that should express its composition }" I might have chosen substances with even more complicated formulas than this; but could Mr. Newlands have given me a short and easily to be remembered term for albumen, and which should, as lie says, "express its composition," I should have been quite content, and almost ready to become one of his disciples. Instead, however, of attempting to answer my question,