80 HAMILTON HARTRIDGE ON 



chromatic difierence of magnification, and was found to improve 

 slightly the spherical correction. 



This method appears to be exactly analogous to that described 

 by Ainslie (5). The fact that the lens in my case was over- 

 corrected chromatically does not constitute an essential difference. 



The images given by this complete condenser system were 

 found to be satisfactory; the comparison between the different 

 methods of illumination was therefore proceeded with. 



Section IV. 



Experimental Comparison between Different Methods 

 OF Illumination. 



The three alternative methods used were : 



(1) Critical illumination in which an image of the illuminant 

 is focused into the slide. 



(2) First alternative in which an image of the illuminant is 

 focused into the lower focal plane of the condenser (called by 

 Ainslie method A). 



(3) Second alternative in which a piece of opal glass is placed 

 at the lower focal plane of condenser, the opal glass being illu- 

 minated from behind by a suitable light-source (Sir A. E. 

 Wright's method). 



The apparatus was arranged so that the change from one to 

 another illuminant could be rapidly effected, at the same time 

 retaining precise adjustment. 



The following tests were applied : 



(1) The resolution of a replica grating by Thorpe of 14,000 

 lines to the inch. A Btiker objective of 40-mm. focal length was 

 used, above the lens being mounted a Davis diaphragm. 



(2) Pleurosignia augulatum into both lines and dots (40,000 

 lines or dots to the inch). A dry 4-mm. Holoscopic objective 

 0'95 N.A. was used, immediately above its back lens being 

 mounted a Davis diaphragm. 



(3) Amphipleura fellucida into both lines and dots (96,000 

 lines to the inch). A Zeiss apochromatic objective of 1*4 N.A. 

 was used. No Davis diaphragm was employed. 



The following method was used in making the comparison. 

 With the objective carefully focused on the slide, the iris dia- 

 phragm of the condenser and the Davis diaphragm (in the case 

 of the 40-mm. and 4-mm. objectives only) were adjusted so that 



