THE SOLAR MICROSCOPE. 
1. 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 
sunshine, and a room haying a suitable aspect, being conditions 
indispensable for its performance. But by the substitution of the 
oxy-hyarogen light, and more recently still, ot the electric light, 
the utility and pleasure derivable from this instrument of popular 
illustration have been immensely extended. 
2. The principle of the solar microscope is the same as that of 
the magic lantern, and we must, therefore, refer the reader to our 
Tract upon that subject, for many details, to save the necessity of 
their repetition. The instrument consists of two parts, essentially 
distinct one from the other: 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. To illus¬ 
trate 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 considerable part of it being lost in various ways in 
passing from the object to the screen. , The necessity, there¬ 
fore, for very intense illumination in this instrument must be 
evident. 
3. 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 100 to 144 
times in its superficial dimensions, would be utterly insufficient, 
if it were rendered 1600 times more feeble. 
4. 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 dis¬ 
tance from the foeus, that the entire object shall be illuminated by 
them. In fact, the rays may be considered as forming a cone 
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