Critical Microscopy. By E. M. Nelson. 289 



object-glass as a rough means of determining this angle of deflec- 

 tion, and hence the fineness of the grating (apart from any resolu- 

 tion of it) before the Abbe theory had been published. This test 

 may be applied not only to rulings, but also to diatoms, such as 

 Naviculacex, Pleurosigma, Nitzschia, Schizonema, etc., but of course 

 not to Aulacodisci, Ewpodisci, Aulisci, Coscinodisci, Isthmia, Tri- 

 ccratia, etc., for in these latter examples the whole of the back lens 

 is flooded with light, and that is the reason why they are more suit- 

 able than any other objects for testing objectives. 



The examination of bacteria upon a dark ground is a plan now 

 much used ; this I advocated in 1884 at the Quekett Micro- 

 scopical Club for the purpose of " saving the eyes from the woful 

 glare of the direct light, and to enable the objects to be much more 

 easily detected." With this kind of illumination the whole area 

 of the objective is utilised, and those who have selected their lenses 

 by a bacterial test and a small cone now find that their lenses 

 break down with this dark -ground illumination, so they are having 

 stops made to cut off the fluffy margins of their lenses so that they 

 may use them with reduced apertures. 



June loth, 1910 



