E. M. NELSON ON DARK-GROUND ILLUMINATION. 205 



movement is already slowed down by the magnifying power of 

 the substage condenser ; a slower motion than this is not required. 

 If a rack is placed anywhere it should be put to the slotted arm 

 which carries the tubular post of the bull's-eye, to enable the 

 flame to be accurately focused. It is curious that rack work 

 has never been placed where it is wanted, but only where it is 

 not in the least needed. 



Method of obtaining a Dark Gi'ound, It is a painful sight to 

 see microscopists fumbling about with their lamps and micro- 

 scopes by the half-hour trying to hit off by some lucky chance a 

 proper illumination of their object. Of course there should be 

 no luck nor chance about it. If the apparatus is correct, the 

 proper illumination should be obtained in sixty seconds ; if it is 

 not obtained, then the apparatus is at fault, and should be 

 changed. For if the proper method is employed in the first 

 instance the best will be got out of the apparatus, however bad it 

 is, and nothing further can be done by hours of fumbling. 



The first adjustment is to place the bull's-eye at right angles 

 to the edge of the flame. The second is to adjust its height 

 so that the horizontal optical axis of the bull's-eye cuts the 

 brightest part of the edge of the flame. The third adjustment 

 is to focus the bull's-eye so that a sharp image of the flame 

 is thrown upon the wall, distant, say, about 5 feet. Having 

 adjusted the bull's-eye to the lamp in this manner, their 

 relative positions must on no account be altered. The brightness 

 of this image will depend upon what the bull's-eye is working 

 at, i.e. adopting photographic terminology, //1'4:, y/2, and so 

 on. The smaller the denominator of this fraction, the brighter 

 will be the image. The doublet of minimum aberration which 

 I designed will be found more useful than an achromatic lens, for 

 the following reasons : First, it is sufiiciently aplanatic for all 

 practical purposes ; second, it is sufficiently achromatic, as it is 

 made of glass having a very low dispersive power. 



An achromatic bull's-eye fails because it is not possible to 



