THE CONDKSSKR 91 



objective would not be fully utilized, which would cause 

 defects in the resulting image. On the other hand, if 

 the condenser is lowered below its correct focus, its aperture 

 is again seen to decrease rapidly (and there may be an 

 outer ring due to overcorrection). Hence the condenser 

 must be fairly accurately focused, in order to give its 

 greatest solid aperture. Some degree of undercorrection 

 of the condenser is often found, due usually to wrong adjust- 

 ment; and sometimes to correction by the maker for a 

 distant source of light. In this case, the focus of the 

 condenser for the image of the source on the slide, with 

 the 16-millimeter objective in use, is longer than when 

 the condenser iris is opened out, and the 2-millimeter oil- 

 immersion objective employed. So that, after focusing 

 the source of light with the low-power objective, the con- 

 denser has often to be raised still more to focus the source 

 for the high-power objective. 



If an uncorrected condenser is used with a small source 

 of light, it will be found that it cannot be focused for the 

 whole aperture at once; for the focal planes of all zones 

 are different, so that it can only be focused for one thin 

 zone at a time. And within one zone it can only be 

 focused for one color. Hence a large source of light must 

 be used to fill more or less of the vacant zones. For, 

 with a small source of light, the image of the source on 

 the slide is spread out by the uncorrected condenser, and 

 so loses intensity and aperture. 



With a condenser corrected for spherical aberrations 

 only, the different colors have, of course, different focal 

 planes. Thus, as already stated, the disc of light on the 

 object (from the diaphragm on the source of light) shows a 

 thin, reddish margin (with not too dense a yellow-green 

 screen, such as No. 66) when the condenser is focused for 

 green. This margin changes to bluish-green when the 

 condenser is lowered, and the yellow and red come into 

 focus. Screens Nos. 66 and 56, as already noted, let 

 through a Httle red. The out-of-focus color circle is, 

 of course, more spread out than the color in focus. Hence, 



