288 



EDWARD M. NELSON ON 



angle of 45° with the base would represent an inch of N.A. 0*1, 

 a 1/2 inch of N.A. 0-2, a 1/3 inch of N.A. 0-3, and so on— a 

 ratio of course ridiculously small.) 



When we plot the ratios suggested by biologists and medical 

 men, who demand full apertures for, say, the 1 in. (with short 

 tube 2/3rd in.), a lens they use, but for their medium powers low 

 angles for penetration, etc., we shall not find a smooth curve, but 

 a zigzag line, like a flash of lightning. All that a microscopist 



has to do is to plot the curve, and he will have no hesitation in 

 appraising the value of a proposed scheme, as to its soundness 

 or worthlessness, for he will see it at a glance ; remembering, of 

 course, that the more the line inclines to the horizontal the greater 

 will be the relative apertures, and, conversely, the more to the 

 vertical the smaller they will be. 



My new curve, practical not theoretical, has been based upon an 

 optical index, diminishing in quantity as the powers increase ; 

 and you may be quite sure that all microscopists of experience will 

 assent to the soundness of this principle, for they know that 2-in. 

 and l|-in. objectives with large optical index can be made with 



