1 84 OPTICAL PROJECTION 



accomplished by having a spare alum-cell with a plane second 

 side, instead of the concave lens. At all events, it seemed 

 desirable to provide the widest possible range in the optical 

 manipulation of substage condensers. 



The rays which reach the substage condenser s c are thus 

 approximately parallel. They really scatter or diverge very 

 appreciably from the parallelising lens p, owing to the size 

 of the radiant, every point in which necessarily sends out its 

 own bundle of parallel rays from P. This is the great diffi- 

 culty in increasing the light by a more powerful jet ; which 

 latter increases the luminous surface, but not the intensity in 

 the centre, which alone can be condensed and made available 

 on a small microscopic object. Some of this scattered light 

 could be saved by allowing the rays from condenser to focus or 

 cross, and then parallelising them by a convex lens ; but an 

 approximately parallel beam all along the barrel of the in- 

 strument has so many advantages as to far outweigh this in 

 practical work. Where however it is absolutely certain 

 polarised light will never be used, the distance from p to s c 

 may be shortened with some slight gain in light. 



In the parallel beam of light is placed the substaye con- 

 denser s c, winch focusses the rays on the slide by the rack 

 R'. For low powers this condenser consists of one lens, 

 which will brightly and evenly illuminate a slide of fully 

 1 inch diameter for a 2^-inch power, while the same lens, 

 racked back, answers for all powers as high as T ^ inch. 

 Above that a double combination is preferable, and for 

 ^-inch or -J-inch powers, one of these is made achromatic, 

 or rather, with sufficient over- correction to correct the other 

 lens as well. For immersion lenses, this last form of con- 

 denser is supplemented by a third very small lens of very 

 short focus ; the whole then closely resembling a Kellner 

 eye-piece surmounted by a small hemisphere of crown, which 

 was kindly suggested to me for such work by Dr. R. L. Maddox. 



