Whatever idealists may assert, there

Whatever idealists may assert, there Post Date: Sat, 2 Aug 2008 0:58:30 +0000
Whatever may be its nature or our ignorance on this point, there is in reality something which corresponds to our idea of extension. Whoever denies this truth must be content to deny every thing except the consciousness of himself, if indeed he does not experience doubts even of this too. Whatever idealists may assert, there is not, nor ever was a man who in his sound judgment seriously doubted; the existence of an external world.

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1. - But, in fact Post Date: Tue, 5 Aug 2008 0:20:57 +0000
To illustrate this, let us suppose that the object exhibited is an insect, a quarter of an inch in length, and that it is magnified 40 times in its linear dimensions, the length of the optical image will then be 10 inches, and its surface will be 1600 times greater than that of the object. The light, therefore, which illuminates the object, supposing the whole of it to be transmitted to the optical image, being diffused over a surface 1600 times greater, will be 1600 times more faint. But, in fact, the whole of the light never is transmitted, a consider- able part of it being lost in various ways in passing from the object to the screen.

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2. - 528 Illuminating apparatus Post Date: Tue, 5 Aug 2008 0:34:56 +0000
The necessity, therefore, for very intense illumination in this instru- ment must be evident. If these conditions were not borne in mind, it might appear that a magic lantern might be converted into such a microscope, by merely increasing the magnifying power of the lenses ; but the light of the lamp, which is sufficient to illuminate a picture mag- nified 10 or 12 times in its linear, and, therefore, from loo to 144 times in its superficial dimensions, would be utterly insuf- ficient, if it were rendered 1 600 times more feeble. 528 Illuminating apparatus.

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3. - In fact, the rays may Post Date: Tue, 5 Aug 2008 0:45:51 +0000
The illuminating apparatus of the solar microscope consists of a large convex lens, upon which a cylindrical sunbeam of equal diameter is projected. This lens causes the rays of such a sunbeam to converge to a point, and they are received upon the object to be exhibited before their convergence to a focus, and at such a distance from the focus, that the entire object shall be illuminated by them. In fact, the rays may be considered as forming a cone which is cut at right angles to its axis by the slider upon which the object is fixed.

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4. - Thus, for example, if Post Date: Tue, 5 Aug 2008 1:00:38 +0000
Let c c, fg 258, be the condensing lens ; let F be the focus to which the ravs would be made to converge, but being intercepted by the slider s s, they are collected upon the small circular opening o o in the slider, and in this circular opening the small microscopic object to be exhibited is mounted between two thin plates of glass. Now, it is evident that the intensity of the light thus projected upon the object, will be greater than that with which it would be illuminated without the interposition of the lens c c, in the exact proportion of the surface of the lens c c to the surface of the circular opening o o. Thus, for example, if the diameter of the lens c c be 5 inches, and the diameter of the opening o o half an inch, the diameter of the lens will be 10 times, and, therefore, its sur- face 100 times greater than that of the opening o o.

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5. - The cone of rays proceeding Post Date: Tue, 5 Aug 2008 1:15:14 +0000
In that case the object would be illuminated with a light just 100 times more brilliant than if the suns light fell directly upon it, without passing through the lens cc. It is found convenient in some cases to condense the light by means of two lenses. The cone of rays proceeding from c c might be received upon another condensing lens, by which its convergence might be increased.

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6. - The lens c c Post Date: Tue, 5 Aug 2008 1:26:29 +0000
The advan- tage of this arrangement is tbat the distance of the object from cc, and therefore the length of the microscope, is rendered less than it otherwise would be. There is, however, one practical inconvenience to be guarded against in this arrangement. The lens c c, which condenses the suns light upon the object, also condenses its heat, and if the same object be exposed in the instrument for any considerable time, it would thus be injured or destroyed.

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7. - It happens that water Post Date: Tue, 5 Aug 2008 1:45:21 +0000
This inconvenience may be obviated by the interposition of certain media, which, while they are pervious to the suns light, are impervious to its heat ; such media are said to be athermanous. By the interposition of such a medium, the object may be pre- vented from receiving any increased temperature whatever. It happens that water, which is the most convenient medium for this purpose, is very imperfectly pervious to heat, and is ren- dered almost completely athermanous by dissolving in it as much alum as it is capable of holding in solution.

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8. - 529 Magnifying apparatus Post Date: Tue, 5 Aug 2008 2:04:27 +0000
The object, there- fore, is perfectly protected from the effects of heat, by placing between the slider and the condensing lens a cell, consisting of two parallel plates of glass, fixed at about an inch asunder, and filled with such a saturated solution of alum. The light inter- cepted by this is altogether inconsiderable, while the whole of the heat is stopped by it. 529 Magnifying apparatus.

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9. - The magnifying powers used Post Date: Tue, 5 Aug 2008 2:21:43 +0000
The magnifying part of the solar microscope consists of an achromatic lens, or combination of lenses, of very short focal length ; this being brought before the object, at a distance from it a little greater than its focal length, will produce a highly magnified optical image of the object, upon a screen placed at a proper distance before it. In the case of the magic lantern, it is not indispensable to incur the expense of achromatic lenses, and even the expedients to correct the spherical aberration are but little attended to. The magnifying powers used in that instrument not being great, and the object exhibited not requiring extreme accuracy of delinea- tion, the expense which would be incurred in producing large lenses free from the aberrations is not necessary.

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10. - As in the magic lantern Post Date: Tue, 5 Aug 2008 2:34:49 +0000
But in the case of microscopic objects, where great magnifying powers are applied, lenses in which the aberrations are not corrected would produce images so confused and indistinct as to be alto- gether useless. Achromatic combinations, therefore, in which the spherical aberrations are also corrected, are in this case indis- pensable. As in the magic lantern, the same lenses may be applied, so as to produce different magnifying effects.

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