136 THE AMERICAN MONTHLY [May 



with a simple sub-stage ring are only approximately cen- 

 tered, but the mounting of the condenser is generally 

 slightly elliptical, and by rotating the condenser an ap- 

 proximately central position can generally be obtained. 

 The value of a proper centering sub-itage is now evident, 

 and the simplest way of centering the condenser is to 

 make a minute ink-spot in the centre of the top lens, and 

 centre accordingly, afterwards wiping the ink-spot oflF. 

 Some few condensers were formerly fitted with a small 

 brass perforated cap for this purpose. Another method is 

 to focus the image of the iris diaphragm when practically 

 closed, and centre accordingly ; but a little practice ena- 

 bles the microscopist to centre his condenser without any 

 of these aids. 



It may be advisable here to mention that almost all ob- 

 jectives differ in the centering of their mounts, and there- 

 fore with high-power work the centering of the conden- 

 ser will vary for each objective. A rotating nose-piece 

 also disturbs the centering of the objectives, and conse- 

 quently of the condenser, as no no«e-piece is construct- 

 ed with sufficient accuracy to ensure absolute truth in this 

 respect. We think we have now made plain the princi- 

 ple which underlies the use of the condenser, and which 

 amounts really to this — that the light and image rays must 

 coincide. If the condenser be adjusted so that its focus be 

 beyond or below the object on the stage, the rays of light 

 will not coincide with the focus of the objective, but will 

 cross, and the result will be an imperfect image. 



There is one other point that may be mentioned in con- 

 nection with the focussing of the condenser. So far, we 

 have purposely assumed that the source of light has been 

 that of a lamp, which gives practically parallel light. In 

 "Carpenter on the Microscope" it is rightly pointed out 

 that ordinary daylight, owing to its diffusion, does not 

 give parallel light, though a window acts to some extent 

 as a limiting diaphragm. The focus of the condenser dif- 



