SMITH, ON MICROSCOPIC ILLUMINATION. 143 



If my previous paper was wanting in courtesy, as the 

 Eev. E. O'Meara seems to think, I can only say that it was 

 unintentional, and beg to apologise for it ; my only desire 

 was to protest against the addition of so many " new 

 species,^^ their claim to that position (in my opinion) being 

 more than doubiful. I could, if I thovight it desirable, 

 publish a score or two of new species, if the fact of their 

 appearing different to any hitherto published is all that is 

 necessary to constitute a new species. 



Microscopic Illumination. 

 By Edwin Smith, M.A. 



It is often difficult to obtain an equally illuminated field 

 for both eyes when a half-inch object-glass is employed with 

 the binocular. The prism causes the field to be darkened 

 on opposite sides for the two tubes of the body. This defect 

 becomes more apparent when the lenses of the object-glass 

 are further separated from the prism by the additional thick- 

 ness of the nose-piece. Diffusing the light with ground 

 glass partly remedies the defect, but not entirely ; moreover, 

 diffused light is not suitable for many objects, where definite 

 shadows are desired for the purpose of displaying structure. 

 I find, however, that an achromatic combination with wide 

 aperture as condenser, and a half-inch mounted in short 

 cells, completely satisfy the conditions of the ju'oblem, and I 

 am now able to employ the half-inch biuocularly with per- 

 fect ease, by night or day. 



Double diaphragm. — To the single diaphragm with which 

 my Webster's condenser is provided, I have added a second 

 plate, revolving close behind the former, and perforated with 

 various stops. By having a large opening in each plate, the 

 stops of either can be brought into play at the choice of the 

 operator, giving a vast range of modifying power, both for 

 dark-ground and transparent illumination. I find the 

 double diaphragm so exceedingly convenient that I wonder 

 it is not always supplied by the makers, the additional cost 

 being a mere trifle. 



Exclusion of incident light. — When viewing transparent 

 objects it is generally important to shade off the incident 

 light. For this purpose I have found much satisfaction in 

 the use of small blackened cardboard tubes, made to slide 



