Monthly MicroscopicalT 

 J jurnul, June 1, ISTO. J 



of High-poiver Definition. 



311 



From tliis Table it will be seen that by reducing the detailed 

 experiments to one uniform standard test of angular visibility, at 

 a power of 1000, the order of merit in which the performance of 

 the glasses should be placed is dependent upon the closest lines re- 

 solved, or rather counted. 



The XlXth band resolved at 183 by the -^ Powell aud Lealand. 



„ XVth „ ,, 



„ Xllth „ 



„ Xllth „ „ 



„ Xllth „ 



11 Xth „ „ 



„ IXth 



„ Vlllth „ 



,. Vlllth „ 



In this result it will be seen that the bands arrange themselves 

 in the order of difficulty of resolution. The XlXth band, at 1000 

 diameters, closely resembles 7' Andromedae, the severe double star 

 so difficult for telescopes. 



It will be extremely interesting to hear that our American 

 brothers can resolve the striae of known diatoms, having them 

 arranged from 120,000 to 150,000 lines to the inch. 



The writer has found in practising upon Nobert's lines some 

 curious phenomena with a Powell and Lealand xVth immersion 

 {■^oih. converted). Spurious lines can easily be formed by oblique 

 light so as to actually represent the empty spaces between the bands 

 as filled with divisions ! 



In some cases a false diffraction line exactly divides a pair of 

 lines so as to make three instead of two. 



* These lines seen at 100" -76 satisfactorily by Mr. StoJder and Jlr. Greeiileaf, 

 p. 136, 'Quart. Journal Microscopical Science,' July, 1868, quoted supra. 



