THE PROJECTION MICROSCOPE 183 



above. The loss of light does not, I think, exceed 10 per 

 cent, when thus constructed, but the edge of the field is not 

 quite so well illuminated. 



In the convergent cone of rays from the lantern condenser 

 is placed a parallelising plano-concave lens P, giving an ap- 

 proximately parallel beam of about 1^ in. diameter. This 

 lens is of highly dispersive glass, and therefore to a large 

 extent corrects the chromatic effects of the lantern condenser. 

 In the same position, nearly, is placed the alum trough A. 

 I found it advisable to employ a full inch of alum solution, 

 and, in addition, to form the second side of the cell of a double 

 plate of glass, the two cemented together by Canada balsam. 

 This layer of balsam absorbs any special balsam-heating rays 

 which get through the alum. 1 With these arrangements 

 protection from heat is perfect. Less than this is not real 

 protection ; for the heat in the conjugate focus of a good lime- 

 light is sufficient to ignite black paper. For ordinary purposes 

 I cement the concave lens with balsam on the alum trough, 

 thus making the lens itself the second of the two glass plates. 

 By this expedient the loss of light at two reflecting surfaces 

 is avoided. 



From the parallelising concave lens to the stage is about 

 5 inches. Less than this would suffice for mere focussing 

 purposes, with plain work only ; but this distance is not 

 enough to produce much loss of light by scattering, while it 

 allows of a really good-sized polarising prism being introduced 

 when necessary, such as will show a polarised slide of J in. 

 diameter none too much for rock sections. Also it appeared 

 to me, from such experiments as I could make, that some 

 achromatic condensers gave more light and worked better 

 when not placed in parallel light, but just after allowing the 

 rays to cross from the lantern condenser, without parallelising 

 them at all. I therefore allowed for this, which is easily 



1 I should remark that I saw a layer of balsam employed to protect balsam 

 elides years ago, but am not now certain by whom. 



