172 THE UiSE OF THE MICROSCOPE 



tected from direct sunlight, and kept in a box when not in 

 use. They do fade in direct sunhght. 



4. Testing the Condenser. — The achromatic and apla- 

 natic condenser must be in adjustment for (a) distance of 

 lamp, (6) thickness of sUde, and (c) the particular immer- 

 sion fluid regularly used for it. A condenser adjusted for 

 parallel rays and oil immersion can be adjusted for water 

 immersion and a definite slide thickness, by putting the 

 lamp at a certain distance, and centering a suitable achro- 

 matic lens close below the condenser. Half or the whole 

 of an appropriate rapid rectiUnear photographic objective 

 will sometimes suit. The aperture can then be further 

 adjusted by observation of the light circle on the back of the 

 high-power immersion objective, with a 1.0-millimeter 

 slide, while a 3-millimeter diaphragm is on the source of 

 hght, so that the Kghted area of the object is equal to or 

 less than the field of the eyepiece. This light circle on the 

 back of the objective should be as large and uniform as 

 possible, and show no marginal ring when the condenser is 

 raised or lowered. If such a ring appears, the lamp distance 

 is to be altered until it vanishes. 



5. Testing the High-power Objective. — This may be 

 done (after the condenser is corrected) by observing the 

 definition and resolution of a specimen of Surirella gemma 

 in hyrax, on which the incandescent lamp filament is 

 focused, with dense green screens between; or by observing 

 the definition with direct and obhque light of the edges of 

 the silver bands in the Abbe test plate; or by observing 

 out-of-focus bright particles in the dark field obtained 

 by the regular or special condensers for objectives of all 

 apertures, up to 1.3 (Siedentopf's method). This latter 

 is the "star" test, as described by Coles. 



6. Testing the Eyepiece. — It is usually sufficient to see 

 that the lenses are free from smears or film. Spots on the 

 eyepiece lenses move on rotation of the eyepiece, while 

 those on the upper prism surfaces of the binocular do not. 

 The diaphragm should not have shifted in the eyepiece tube, 

 and its edge should be sharp to normal distance vision. 



