208 OPTICAL PROJECTION 



The instrument as thus modified, and fitted with polarising 

 apparatus, is shown in fig. 103, which also shows Mr. Brockie's 

 optical lamp. 



In using the electric light, I very soon found that there 

 was some difference in the effect of the substage condensers. 

 Owing to the smallness of the radiant, and its greater distance 

 from the condensers, the light is brought by the same lenses 

 into a much smaller area ; and hence the same sub -condenser 

 used for low powers, answers perfectly well up to T % power as 

 well. In the higher powers also, lower angles may often be 

 employed. This likewise tends to increase penetration and 

 definition. What a microscopist understands by * high reso- 

 lution ' is quite unattainable with the lantern, since the dots 

 on P. Angulatum, even magnified up to 5,000 diameters in 

 nearly black and white, would be invisible a very few feet 

 away from the screen. 



104. The Electric Lamp. On this head I need only refer to 

 the preceding chapter. It was all-important to get rid of the 

 double radiant, and this is fully accomplished in Mr. Brockie's 

 lamp there described, while all necessities of centering and 

 focussing are met by the screw-motion table on which the 

 lamp stands. 



105. High-power Demonstrations. For high-power work, 

 especially biological, I would repeat that a permanent screen 

 or surface of fine plaster of Paris, smooth, and kept carefully 

 whitened, is of the greatest assistance. The dark parts of the 

 image are as dark upon it, while the bright parts are much 

 brighter ; hence that contrast of shades is heightened whose 

 dilution is one of the great difficulties we encounter. Such 

 screens are used in German institutions, and enable results 

 to be obtained which otherwise would be beyond the apparatus 

 employed. Such a surface, and opera-glasses for the back rows, 

 will do a great deal to extend the range of detail possible. 



In the majority of cases, working with a good projection 

 eye-piece is decidedly to be preferred. A really fine - or |, 



