riioTOMiCRocK.iriiy .ixd rriiixicti. MiCROscory 107 



iimmTiral .i|K'rtiiri'. As tlu' iiiiimTic.il .iiuTtiirc increases the depth of 

 ()fnctration (i.e., tlie power of the objective to resolve detail simul- 

 taneously at different depths or distances from the objective), and 

 the flatness of the field both decrease, but usually when high resolution 

 is desired flatness of field anfl (lenetration are not of great concern. 

 The value of the numerical aperture varies from about 0.10 in the 

 ver>' low-fK)wer achromatic objectives to 1.40 for the oil immersion 

 apochromats. 



The eyepieces for use with the achromatic objectives are general!)- 

 of the Hu> gens type but those for use with apochromats are termed 

 compensating because of certain corrective measures which they 

 apply to the beha\ ior of this type objective. High-power achromatic 

 objectives and the semi-apochromats may also be used to advantage 

 with the compensiiting eyepieces. The magnifying power of eye- 

 [)ieces ranges from about 4 times to 20 times although another class 

 termed orthoscopic eyepieces may be procured with a magnifying 

 power of 28. These latter eyepieces are generally used with low- 

 power objectives only. A special type of eyepiece known as a pro- 

 jection eyepiece of low magnifying power is used for certain classes 

 of work when photographing with a long bellows extension. These 

 eyepieces have correction collars which must be set to correspond 

 with the bellows extension used. 



Il.lAMINATlOX 



The color of the light used and the illumination of the specimen 

 play a most important part in photomicrography and the behavior 

 of the finest objective will appear very ordinary unless critical il- 

 lumination of the specimen is attained. The illuminant is usually 

 some form of arc lamp or metal filament, gas-filled lamp. Both 

 types have their advantages and while many statements may be 

 found derogatory to the use of arc lamps, as a real source of light, 

 the author has found that a smoothly operating automatic arc lamp 

 equipped with suitable carbons is capable of yielding results of the 

 highest order. What is needed especially for medium and high- 

 power work is a pioint source of light (or approximately so) of great 

 brilliancy; capable of being smoothly and uniformly controlled so 

 that the luminous end of the positi%e carbon will not fluctuate back- 

 ward and forvvard within wide limits. Most automatic arc lamps 

 are designed for a certain direct current value, usually about five 

 amperes and unless the current rating is closely adhered to in practice 

 the operation is apt to be irregular. Sputtering and irregular feeding 



