One right line may

One right line may Post Date: Fri, 1 Aug 2008 23:25:37 +0000
26 Although extension is essentially composed of parts, there is in it something fixed, unalterable, and, in some manner, simple. There may be more or less extension, but not different kinds. One right line may be longer or shorter than another, but its length is not of a different species.

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1. - To prevent this Post Date: Mon, 4 Aug 2008 23:24:16 +0000
The points of the pencils being nearly in contact, the light will be produced in the manner just explained. Although the charcoal does not, properly speaking, undergo combustion, it is gradually wasted, and when the points would thus become sepa- rated, the current would be suspended, and, therefore, the light would cease. To prevent this, and to maintain the illumination, an apparatus con- sisting of clockwork is provided in the case p, by which the charcoal pencil, a, is kept nearly in contact with the pencil, c.

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2. - This is accomplished by placing Post Date: Mon, 4 Aug 2008 23:37:40 +0000
The clockwork is so constructed that its motion is governed by the current. M Dubosc has contrived means, by which a single electric light will serve to illuminate at the same time two lanterns, placed side by side for exhibition. This is accomplished by placing the light between two reflectors, so inclined that each reflects it in the direction of the axis of one of the lanterns.

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3. - But by the substitution Post Date: Mon, 4 Aug 2008 23:48:16 +0000
As an instrument for popular and general instruction, the solar microscope holds a high place. Until recently, its use has been restricted in these climates, by the circumstance of bright sun- shine, and a room having a suitable aspect, being conditions indis- pensable for its performance. But by the substitution of the oxy- hydrogen light, and, more recently still, of the electric light, the utility and pleasure, derivable from this instrument of popular illustration, have been immensely extended.

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4. - Since it is desired Post Date: Tue, 5 Aug 2008 0:03:17 +0000
527 The principle of the solar microscope is the same as that of the magic lantern already explained. The instrument consists of two parts, essentially distinct one from another : the first, the illuminating ; and the second, the magnifying part. Since it is desired to exhibit a very enlarged optical image of a very minute object, and since the light which is spread over the image can only be that which falls on the object, it is evident that the brightness of the image will be more faint than that of the object, in the exact proportion in which the surface of the former is greater than that of the latter.

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5. - 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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6. - 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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7. - 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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8. - 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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9. - 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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10. - 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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