THE CONDENSER 85 



(Kochler'y inclliod). This caii mIso he doiu*, as already 

 stated, by putting a wcll-illuiuiiiaicd disc of ground glass 

 in the diaphragm carrier (Zeiss, Hartridge). 



The Aspheric (Aplanatic) Condenser. — An improvement 

 in the uncorrected three-lens immersion condenser has been 

 effected by grinding (figuring) one surface of the lower 

 lens approximately to the aspheric curve found empirically 

 to correct the spherical aberrations of the three lenses 

 for yellow-green light, so that all zones of the condenser, 

 when tested, are seen to focus accurately together. Such a 

 condenser is now made by two or more firms (Fig, 18). 

 It gives somewhat more light, in the writer's experience, 

 than the corresponding achromatic condenser, when used 

 with a deep yellow-green filter; and, with a small source, 

 has an aperture, when corrected and oil immersed, of over 

 1.3, which is as high as is needed. It can be used advanta- 

 geously with fairly deep green screens, such as Wratten 

 Nos. 57A and 58; and, doubtless, with monochromatic 

 fight, such as the mercury-vapor lamp screened for all 

 but the bright-green line. Used with the Wratten filters 

 Nos. 66 or 56, the focus of the green is of course shorter 

 than that of the small amount of red also transmitted; 

 and this causes the image of the source at the focus of the 

 green to be surrounded by a fringe of red. This condenser, 

 corrected for use with lamp at 25 centimeters (and also as a 

 water-immersion) by centering an appropriate achromatic 

 meniscus below it, is, in the writer's opinion, when used on 

 difficult objects, nearly equal to the achromatic-aplanatic 

 condenser; both being used with a rather dense yellow-green 

 screen, such as Wratten No. 58. It allows of good results 

 even with apochromatic objectives; and, with a deep-green 

 screen, is about as good as is required for high-power work 

 with achromatic objectives. It should, however, be cor- 

 rected by the makers for a lamp at 25 centimeters, and for 

 water immersion. 



The Achromatic and Achromatic-aplanatic Condensers. 

 An achromatic condenser may be corrected only for 

 rays indefinitely close to the optical axis (paraxial rays). 



