APLANATIC CONDENSERS 105 



beam of light is required, or with the convex side towards 

 the incident light when it is used to converge a parallel 

 beam. 



A double convex lens is also frequently used as a con- 

 denser. Suitable sizes are 2" to 4J" in diameter, with 

 focal lengths between 3" and 5". The Kohler lenses of 

 Messrs. Zeiss, of this type, are specially constructed 

 for condensing purposes. Their use is described on 

 pages 134 and 150. 



Corrected Auxiliary Condensers. Many workers con- 

 sider that it is necessary to have the auxiliary condenser 

 corrected with very considerable accuracy, and Messrs. 

 Watson supply an achromatised aplanatic combination, 

 computed by Mr. Conrady, and shown in Fig. 3. The 

 lens is of very short focal length, and consequently has 

 to be placed very close to the radiant to give a parallel 

 beam of light ; glycerin is therefore used between the 

 component lenses instead of balsam, which would melt ; 

 and to prevent bubbles forming in the glycerin it is 

 essential to use a cooling trough between the condenser 

 and any light source that gives much heat. Although 

 its diameter is only 2" it gives a much, more intense 

 illumination than a considerably larger uncorrected lens. 



Aplanatic Condensers. Intermediate between this very 

 accurate lens and the simpler condensers lie the aplanatic 

 condensers now so largely used, and of which the 

 doublet designed by Mr. Nelson (Fig. 32), is a well- 

 known example. These are also small lenses of about 

 2" diameter, and although not perfectly aplanatic 

 they give very good results ; as there is no liquid 

 or balsam between the component lenses a cooling 

 trough need not be used to protect them. In our experi- 

 ence the chief advantage presented by the highly cor- 

 rected condensers over simpler but well-made types, 

 lies in the shorter exposures they allow, and this is 

 sometimes an important consideration. The photo- 



