144 THE MICROSCOPE IN THE 



clear that the light which falls on the half-inch circle will be diluted 

 on the screen to i — 46,656th part of the brightness of the light 

 where it met the object, and if, as must be the case in most minute 

 objects, the area of the enclosing circle does not exceed a quarter 

 or a one-eighth of an inch, the light on the screen will not exceed 

 the 1-186, 624th or the 1-746, 496th part of the original bright- 

 ness. It is, therefore, needful that the original brightness of the 

 light, as it meets the object, be increased as the magnifying power 

 is increased. 



The rays of light that proceed from the incandescent lime are 

 divergent, passing forward in straight lines in every direction, and 

 only such portion of them as we can compel to change their direc- 

 tion are of any use to us. The first thing, therefore, is to convert 

 these diverging rays into a parallel beam of light. This may be 

 effected by means of a concave mirror or a convex lens. For the 

 purpose we have in hand, a lens will be the more convenient. 

 The action of a second lens changes this parallel beam into a 

 convergent beam, and in this convergent beam the object is 

 placed, its position being dependent on its diameter. The final 

 result on the screen will depend to a great extent on the accuracy 

 with which this conversion of the rays is accomplished. The 

 source of light must be concentric with the optical axis of the 

 lenses, and the lenses must be of such a character, both as regards 

 their transmitting and their refracting powers, as to arrest as little 

 light as possible, and conduct it forward with the least possible 

 dispersion. Theoretically, there ought to be no difficulty in 

 bringing the light which issues from the lime into a cone with a 

 very fine apex ; but in practice it is very difficult. There is a 

 large amount of light reflected from the various surfaces of the 

 lenses — the light itself is not a simple point — and several other 

 causes contribute to make the cone of rays very imperfect. And 

 yet the adequate illumination of the smaller objects depends 

 almost entirely on the perfection of this cone. 



An arrangement should be made whereby the object to be 

 illuminated can be placed in that part of the cone of light which 

 will best cover it. Thus, the more minute the object, the nearer 

 the apex, and vice-versa. If the cone were perfect, the screen 

 ought to be equally well lighted under all conditions of ampUtude ; 



