508 Dr. Gr. J. Stoney mi Microscopic Vision. 



and putting cr = 0°265 fi, we find 



X= (0-265) x (1-25 + 0-85) 

 =0-56/*, 



which is the wave-length of a ray hetween the lemon-coloured 

 part of the spectrum and the green. We thus learn that 

 light of the above wave-length i& the least refrangible which 

 can help us to see the detail we are looking for. We ought 

 accordingly to exclude all red, orange, and yellow rays since, 

 if present, light of these colours would throw a uniform glare 

 over the plains upon which the detail lies. This condition is 

 best complied with by setting up prisms to limit the range of 

 wave-lengths that we employ. Two two-inch equilateral 

 prisms of dense glass brought- into their position of minimum 

 deviation were found to answer well. The light to be chosen 

 should be of a colour for which the objective is specially well 

 corrected, more particularly as regards aplanatism; and for eye 

 observations it must be in a sufficiently luminous part of the 

 spectrum to supply brightness enough. It should obviously 

 consist of the rays of shortest wave-length that fulfil these 

 conditions ; and on trial the best light was found to be 

 bluish green of about \ = 0*52 fi. This wave-length is 

 nearly that of the great magnesium triplet in the spectrum. 



No collimating-lens was employed to prepare the light for 

 the prisms, but the slit (a coarse one) was adjusted to 

 such a distance from the prisms as enabled the condenser to 

 produce a disk of light in image x of the full size corre- 

 sponding to its grasp, when the condenser was itself adjusted 

 at such a distance from the microscopic object as to produce 

 one uniform colour throughout that disk of light. Upon the 

 slit the image of an incandescent gas-burner had been 

 thrown, which was thus the source of light. The mirror of 

 the microscope was then turned until the colour seen in 

 image x was bluish green. With these arrangements we 

 know that the wave-lengths we are using are in the neigh- 

 bourhood of X=0-52yu-. 



We are now in a position to determine what size of stop 

 it will be advisable to introduce under the condenser, so as to 

 block out most of those direct beams whose diffracted light 

 does not come within the grasp of the objective, and which 

 accordingly would be not only useless but prejudicial. Since 

 \=0'52/a and o-=0"265 fi we find by equation (2) that 

 <? + </ = 1*96. Hence when g has its largest value, which is 

 T25, g' will have its smallest, which accordingly is 



</=Q-71, 



