The

condensing lens must be used as described (Fig. 2). focus of the reflectors may be made to coincide exactly with that of the condensing cups by a small elongation of the tube, (d, Fig. 1). Care should be taken that the tubes containing the reflectors are screwed perfectly home, so that the aperture may be truly opposite the slider-holder, and in the line of the bar; a rase is cut round the tube of reflectors, as a rough guide for bringing the object under the aperture. The shortest tube of reflectors is the most powerful of the set: the focus of its concave reflector is of an inch, its diameter full, and the object requires to be placed about of an inch from the side

of the tube.

The second pair of reflectors is aperture; its focal point is about of the tube.

6

of an inch focus, and of an inch from the side

The third pair of reflectors is one inch focus, and full of an inch aperture; the focal point about of an inch from the side of the tube. The longest tube of reflectors is 2 inches focus, and an inch aperture; the focal point is half an inch from the side of the tube: from its small power, it is that best adapted for the larger objects. The instrument is furnished with three eye-tubes, and, like the tubes of reflectors, the shortest magnifies the most. Either of the eye-tubes can be used with the different tubes of reflectors, observing that it is always best to gain the power by the reflectors. Use, therefore, short reflectors and long eye-tubes.

When the greater powers are wanted, of course the short eye-tubes are to be used. By lengthening the body of the instrument, a great increase of power may be obtained.

In some instances an inconvenience occurs from this mode

of gaining power, as the focus is brought nearer to the side of the tube; of course the light is not then so easily applied to opaque objects.

The powers given by the combinations are nearly as follows. The long tube of reflectors, with the combinations of the three eye-tubes, magnifies from 5.00 to 5.000 times in superficies; the next in length, from 3.000 to 50.000; the next tube from 8.000 to 160.000; the short tube up to 320.000; the elon

gation of the tube produces a further increase up to 1,000,000 times. The most generally useful reflector will be found to be the shortest but one. The shortest one is only requisite to define the most difficult class of lined objects! 10 11 1

"

Particular care should be taken to keep the tubes of reflectors closed when not in use: the metals will not require any cleaning, and should not be removed from their tubes. The glasses of the eye-tubes require to be most attended to, as dust or finger-stains upon them very much lessen the brightness and distinctness of vision. When the eye-tubes are cleaned, only one glass should be removed at a time, and wiped with a soft piece of leather, and then replaced, before another is taken out, to prevent the misplacing of them, as such a simple mistake would render the instrument nearly useless.

The talc between which the objects are placed being of a soft nature, care should be taken not to scratch or soil it with the fingers. Dust is best removed by a camel hair pencil, and stains by a soft piece of leather. It often occurs in cold weather, that, when the instrument is used, it is considerably below the temperature of the room or observer, and, consequently, a condensation of vapour takes place on the glasses of the eye-tubes, which prevents distinct vision, and though wiped off, will still continue to condense from the eye of the observer. Till the instrument has acquired sufficient warmth to prevent such impediments, it is requisite to place it for a short adgrobizno) time before the fire.

9 The most delicate test objects are the lines on the feathers of butterflies or moths' wings, of which there are many gradations; some easily demonstrated, and others more difficult fo be seen, and then only with the most powerful reflectors, and seen to the best advantage by the simple and uncondensed light of the lamp. The light must be so arranged that the rays will pass through them in an oblique direction; also the positions of the object must be attended to, for in some posi. tions not a line will be seen, when a little variation of the light may render them perfectly distinct. The hair of a mouse is a very good test object: it is best seen by daylight; the most difficult parts of which are longitudinal lines in the transparent a Tot ebonC !

I

1

part of the hair, which require high powers. The hair of the bat and seal are also fine tests. The lines on the scales of the diamond beetle, &c., are excellent opaque proof objects. The feet of flies are likewise very interesting. : od sush

It does not become me as a maker of these instruments to say more of them than that they have been compared with the best microscopes now extant, and have given the greatest satisfaction; and that it is the opinion of most observers that. ༠༠༠༠ མ the vision f from truly-executed specula produces a degree of delicate distinctness which no refracting microscope can rival.

Remarks on the Discussion between Mr. Ivory and Mr. Meikle, bus lision the Temperature and Condensation of Air.

SIR III

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you consider the following remarks worthy of a place in the Quarterly Journal of Science, their insertion will oblige 29226ly 90 Your obedient Servant,

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1-979 od) to A CIVIL ENGINEER || //

The laws of heat as connected with air, being a subject of considerable importance as a branch of natural philosophy, as well as in its various applications to useful science, I have been induced to offer a few remarks on the papers lately pub lished on this subject by Mr. Ivory, in the Philosophical Magazine and Annals of Philosophy, and by Mr. Meikle in the Quarterly Journal of Science.

From the high character of Mr. Ivory as a mathematician, I should have been almost inclined to consider any such weighty charges of error brought against him in a manner so unceremonious as those by Mr. Meikle have been, as-undeserving of attention, had not two papers of considerable length appeared from Mr. Ivory in his defence; and as the peculiar manner of his replies seemed to betray strong symp toms that there might possibly exist some grounds for the

accusations, I now find that, after an attentive and impartial examination of both sides of the question, my suspicions have been completely realized.

The two principal points of Mr. Ivory's doctrine which Mr. Meikle impeaches are the following:

"1st. That the heat extricated from air when it undergoes a given condensation is equal to of the diminution of temperature required to produce the same condensation, the pressure being constant.

"2d. That while the ratio of the specific heat under a constant pressure to that under a constant volume is invariable, equal quantities of absolute heat produce equal increments of volume."

As to the first of the above propositions, the point to be settled is briefly this;-whether Mr. Meikle has correctly expressed it by the formula

for if so, the whole train of absurd consequences pointed out by him must inevitably follow. The manner in which it is deduced, given by Mr. Meikle in his last reply, being extremely simple, and had it contained any error, Mr. Ivory could not have failed to perceive it; and instead of writing a reply of six pages in length, consisting, as Mr. M. observes, of little else than a repetition of what he had formerly published, why did he not at once point out the error, which he might have done in little more than as many lines;—the trick, of which he complains so bitterly of having been played upon him, would thence have been exposed, and, consequently, the whole of the argument of his opponent would have fallen to the ground. In short, Mr. Ivory's complete silence on this head affords the strongest presumptive evidence that he was unable to discover any such error to exist, otherwise he would never have shrunk from this his only consistent mode of defence.

With regard to the second proposition, Mr. Meikle seems to admit the first part of it, respecting the inconstancy of the ratio, but denies the second as inconsistent with it, and certainly the

demonstration which he has given to shew this is both ingenious and conclusive. It only remains to inquire into the grounds which Mr. Ivory appears to have had in support of his opinion. Accordingly, on looking over his papers on the subject, what appears to bear most on this point is the following passage in the No. of the Philosophical Magazine for February, 1827.

"If we compare two thermometers, one of air and the other of mercury, which have their scales adjusted to the fixed points at which water freezes and boils, and find that their indications agree for a long range of temperature, we must infer that the supposed principle is true in nature for the whole of the interval." That such a deduction as this should have been made by a profound mathematician, which Mr. Ivory must undoubtedly be allowed to be, seems in no small degree surprising, as nothing appears more clear than that the only inference which can be made from the above acknowledged fact is, that the law of expansion, or, which is the same thing, the variation of capacity for heat, is the same in both bodies, but it affords no means whatever in determining the nature of that law. This may be easily shown by a geometrical diagram in a manner similar to that in which Mr. Meikle has demonstrated the law of variation itself.

Let A B and A'B' be any two similar curves, having asymptotes CD, C'D'; and let the ordinates A D, A' D' represent the respective capacities of the two bodies for heat, the abscissas CD, C'D' the corresponding expansions for any given range of temperature: the points CD, for example, may be considered as the freezing and boiling points of water. Now it is obvious. that if the ordinate AD always express the capacity at the point D of temperature, the space A B C D will be proportional to the quantity of heat employed in raising the tempe