496 DUNCAN J. REID ON CRITICAL ILLUMINATION. 



centre of the field, the rest of the field may be darkened by 

 partially closing the Nelson-lens iris, so as not to fatigue the eye 

 by an unnecessarily large field. This, however, can only be done 

 if critical illumination is being used. 



For oil-immersion objectives of high aperture (above 1*0 N.A.) 

 an imm^ersion condenser, with a N.A. of 1'35 to 140, must be 

 employed, and the lamp and collecting lens moved farther away 

 from the microscope (24 to 36 inches or more), depending on 

 the character of the condenser and on the focal length of the 

 collecting lens. 



For medium powers (1/2 to l/4th inch) the dry condenser of 

 1*0 N.A. may again be used, the lamp and collecting lens being 

 now, however, brought much nearer (6 to 12 inches), with Nelson 

 lens and English form of condenser, and rather farther off 

 than this for the continental form of condenser. 



For low powers (2 inch to 1 inch) a low-power condenser must 

 be employed, or the front combination removed from either the 

 dry or the immersion condenser, and lamp and collecting lens 

 brought as near as from 4 to 8 inches. 



That concludes what I have to say. Of course in the scope 

 of a paper, and in the time at my disposal, there are many 

 things which have to be hurried over or omitted, but I would 

 like, for the benefit of those who may wish more information on 

 the subject, to refer them to Carpenter's work on the Microscope, 

 and more particularly to Dr. Spitta's book on the same subject, 

 where they will find the difficulties of illumination treated in a 

 masterly way, both from a theoretical and from a practical point 

 of view. 



Joum. Quekett Microscopical Club, Ser. 2, Vol, XI., No. 71, November 1912 



