THE MICROSCOPE. 



Fig. 4. 



Images." It follows from what is there stated, that all confusion 

 produced by this cause, can be removed by substituting for simple 

 convex lenses, compound ones, consisting of a 

 double convex lens of crown-glass c c', fig. 4, 

 cemented to a plano-concave lens of flint glass. 



The image produced by such a combination, 

 will be distinct and free from colour, provided 

 that certain conditions be observed in the curva- 

 tures given to its component lenses. 



13. Assuming then, that by such combinations 

 the image presented to the eye-glass is a faith- 

 ful reproduction of the object, in its proper colours, 

 perfectly distinct in all its lineaments, and suf- 

 ficiently amplified, there is still one remaining 

 condition for distinct vision, which is, that this 

 image should be sufficiently bright. It will, 

 therefore, be necessary here, to examine the con- 

 ditions on which its brightness, or illumination, 

 depends. 



In the first place it is very evident that, other things being the 

 same, the illumination of the image will be proportionate to that 

 of the object, and in the inverse proportion of its superficial 

 amplification ; for the light which is transmitted from the object, 

 being diffused over the surface of the image, will be necessarily 

 more feeble as the superficial magnitude of the image is greater. 

 The higher the magnifying power used, therefore, the greater is 

 the necessity that the object should be intensely illuminated. 



But the brightness of the image depends not only on the in- 

 tensity of the illumination of the object, but also on the proportion 

 of the light emanating from each point of the object, which arrives 

 at the corresponding point of the image ; and this, as we shall now 

 show, will depend conjointly on the linear opening, or available 

 diameter of the object-glass, and the distance of the object from it. 

 To make this more plain, let o o', fig. 5, be the object, and L I/ 

 the object-glass. We are to consider that each point of the object 

 is a centre, from which rays emanate towards 

 the object-glass ; thus, for example, rays 

 issuing from the point c, form a cone, of which 

 the object-glass is the base, and of which c is 

 the vertex ; supposing all these rays to pass 

 through the object-glass, and to be refracted 

 by it, they will converge to the point of the 

 image which corresponds to c. 



In the same manner, the rays which diverge 

 from any other point, such as o, likewise form a cone, of which 

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