8l> the AMERICAN MONTHLY [March 



A spieatl slide of /-*. a iK/nhit inn dry on cover is an ex- 

 cellent test. The niininunn power required to see it in 

 dots with a i' axial cone is about 220 dianis. 1 have my- 

 self ^dimpsed it with siitrhtly less, l)ut then the image 

 was very difficult. An old (dieaj) student's \ N.A. *72 

 sliowed it with a nui};uifi(ation of 250. Probably some 

 of the modern cheap senii-ai»ochroniats would do it with 

 less. The Zeiss a})0(diroiuatic \ N. A. Br) dots it easily 

 with a large axial cone. It has even been seen vvitli this 

 fine lens with the 8 compensating eyepiece, P. and L. 

 old achromatic 4-10 N.A. -(14, power, 290, also does it. 

 All modern students" , and ,';, senii-apochromatic or other- 

 wise, should do it also. 



The golden rule for the resolving power of any objec- 

 tive with a •] axial cone of illumination is that they should 

 show a fineness of structure equal to 70,000 mult ii)lied by 

 their X.A. Thus— 



TAliLE I. 



Fineness of Strueture 



N.A. Ixosolved. 



01 ; 7,000 



()-2 14,000 



0-8 -21,000 



0-4 -28,000 



05 iio.OOO 



0-6 4-2,000 



0-7 49,000 



0-8 r,fi,0(IO 



0-y (j;),()Oo 



1-0 70,000 



M 77.000 



l-> 84,000 



i:i !)1,000 



1-4 9S,000 



15 1(1."). (100 



Table IJ. agrees very wtdl with Table 1. It must be 

 remembered that some of the lenses which ai)[)arently do 

 not come up to the rule gave a very strong restdution of 

 the numbers ojjposite '.o them; they therefore would 

 probably have resolved a trifle more, but there was not 

 at hand a slightly finer-marked diatom to test them on. 



