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, hy the circumstance of bright 

 sunshine, and a room having a suitable aspect, being conditions 

 indispensable 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. 



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 focus, that the entire object shall be illuminated by 

 them. In fact, the rays may be considered as forming a cone 



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