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object, as the lens is placed; but as we have not dis tinct vision by the eye at much less than sixty times that distance, or six inches, the image will therefore appear to be magnified sixty times. By such a microscope, we shall see the object as large as it would appear to the naked eye, if it were sixty times as near to it as we usually place an object, which we desire to view distinctly with the naked eye.

A double refracting microscope consists of an objectlens of a small focal distance, and one or two eyeglasses. If the object be placed at a little more than the focal distance of the object-glass from it, it will have its image formed inverted on the other side of the lens, and proportionably large, as both will be seen under the same angle from the object-glass. Now, if an eye-glass be applied to view this image at its focal distance from it, the rays will proceed parallel after refraction through it, to the eye placed in the axis of this lens. This compound microscope will magnify an object in the compound ratio of the object's distance from the lens to the limit of distinct vision to the naked eye, which is about six inches, and of the distance of the eye-glass from the image, to the distance of the object-glass from it. The first of these ratios is evident, from what we have said of the single microscope, where the eye being placed at the object-glass, would see the object as much greater, as it is now nearer to it than the limit of distinct vision: but if the eye be placed at the eye-glass, to view the distinct image, it will now see it as much more magnified, as it is now nearer to it, than when it was at the objectglass. Therefore, when the eye is at the eye-glass, it will see the image magnified in the ratio of its distance from the image, to the image's distance from the ob

ject-glass, which is the last part of the ratio. And whatever magnitude the image will appear to have to the eye placed at the eye-glass, the same it will have to the eye any where in the axis of it, because the image is at its focal distance.

In some microscopes, a plano-convex lens is interposed between the object-glass and the distinct image, so that the rays may suffer a second refraction before they are converged to a focus. This will contract the image and make it less, but the field of view will be enlarged, and the vision will be more agreeable.

The area of the object-glass ought to be small, because the object being placed so near to it, the extreme rays would diverge so much, if the glass were large, as to render the image indistinct. But by contracting the area of the glass, these very diverging rays are excluded from the picture. But hereby also we render the image obscure, and therefore to increase its brightness, various ways are contrived, such as placing the object in the sunshine, or collecting the light by a convex lens, or a concave mirror, and throwing it upon the object.

A microscope of the reflecting kind may also be made with a small concave mirror. If the object be placed nearer to the mirror, than its principal focus, the image will be behind the mirror, and erect and magnified in proportion to the distances of the object and image from the vertex. But if the object be placed farther than the focus from the mirror, the image will be on the same side, inverted, and magnified in the same ratio mentioned above.

TELESCOPES.

REFLECTING TELESCOPES, of the Gregorian form, are made with two concave mirrors, placed at a little more than the sum of their focal distances apart, and having their reflecting surfaces turned to each other. The larger mirror is placed at the bottom of the tube of the telescope, and has a central hole in it, to admit the rays to pass through it, which are reflected from the small mirror. When the rays come from a distant object, and fall upon the large mirror, they are reflected to its focus, where they form an inverted image of the object; and diverging from that image, considered as an object, they meet with the small mirror placed in the axis of the tube, at a little more than its focal distance from the image, by which they are reflected back through the central hole of the large mirror, converging to a focus in the small tube, which contains an eye-glass or two, where a second image, is formed, inverted with respect to the first, or erect with respect to the object. This secondary image is lessened a little by the rays passing through a planoconvex lens before the image is formed, and then viewed through an eye-glass of a very short focal distance, placed at its focal distance from the image. The power of magnifying in this telescope, without the plano-convex lens, is compounded of the ratio of the focal distance of the large mirror to the distance of the first image from the small mirror, and of the distance of the second image from the small mirror to the focal distance of the eye-glass. Or, in other words, multiply the focal distance of the large mirror into the distance of the last image from the small mirror, and divide this product by the rectangle of the focal

distances of the small mirror and eye-glass, and the quotient will give the magnifying power of the teles

cope.

Instead of the second concave mirror, if a plane speculum, placed at an angle of 45 degrees with the axis, were interposed to receive the rays from the large mirror before they arrived at their focus, they would thereby be reflected towards the side of the tube, where the image would be formed, which might then be viewed by an eye-glass in the side of it. This is the Newtonian form of the reflecting telescope; and as there is but one image formed in it, its magnifying power will be in the ratio of the focal distance of the eyeglass to the focal distance of the object-mirror.

The refracting telescope consists of one object-lens of considerable focal length, and one eye-glass, placed at the sum of their focal lengths from each other. When rays from a distant object pass through the object-glass, they are collected into its principal focus, where they form an inverted image of the object, which image is viewed by an eye-glass placed at its focal distance from the image, that the rays which diverge from it may fall parallel upon the eye. The magnifying power of this telescope is in the ratio of the focal distance of the eye-glass to that of the objectglass. Because to an eye at the object-glass, or at the same distance from the image as the object-glass is, it would appear under the same angle with the object; and if the eye be placed nearer to it, as for example at the eye-glass, it would appear as much greater as the eye is nearer. Therefore if the eye were placed at the station of the eye-glass, the object would appear as much greater to it, than to the naked eye, as it is nearer to the image at the eye-glass than at the object.

surrounded with a white halo, and upon closing the eyes, the spectrum will appear yellow at first, and soon change into blue, which may continue for some days; as the retina having been so violently stimulated into action falls into various successive spasmodic actions, which may continue several hours, or even induce a temporary paralysis of the organ of vision. Hence the danger of looking too long at very bright and luminous objects.

The direct and inverse spectra may exist at the same time, excited by different or by the same objects. When six inches square of bright Indian pink paper is viewed for some time, upon a foot square of white paper, the green spectrum, viz. the reverse of the pink paper, appears in the center, while the external spectrum is of a pink colour, which is the direct spectrum of the coloured paper. The same will happen when the internal area is white, and the external area is pink colour. While the rays, from the bright internal object, fall with their full force upon the retina, and by fatiguing it, induce the reverse spectrum; many scattered rays from the internal pink coloured paper, fall upon the external parts of the retina, with such force as is sufficient to produce a direct spectrum of it. In the same manner, six inches square of violet paper, viewed on the same white ground, exhibited a yellow spectrum, the reverse of the violet, while the external spectrum was of the same colour, being its direct spectrum, produced as before explained.

If two colours, mutually the reverse of each other, be viewed, one surrounding the other, the scattered rays from the one, which only tend to excite its direct spectrum, coinciding with the reversed spectrum of the other, will make both the spectra more bright and

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