HIGH MAGNIFICATION MICROGRAPHS 185 



to obtain a syst(6inatic centering of the light; the second item, 

 effected by lens F and objective O, is necessary in order to limit 

 properly the image on the plate, and to cut off false light so far 

 as possible. 



There is, in our view, no reason why the parts of this optical 

 system should be differently arranged (sliding of L) when using a 

 plain glass reflector or a prism reflector; nevertheless, on the micro- 

 scope examined, as well as on other Continental microscopes, such 

 a difference has been introduced intentionally. As a matter of fact, 

 it was found quite practicable to obtain correct results, so far as 

 the illumination is concerned, with both kind of reflectors, without 

 any variation, in the position of L (lens F had to be changed). 



T 

 



P 



Fig. 3. 



5. Comparison between Le Chatelier prism and 45° prism. 



The Le Chatelier prism being so constructed that the lens F is 

 formed in the same piece of glass as the reflecting surface P — 

 which, in fact, is made up of two planes giving successive reflection 

 — it was to be expected that it would produce sensibly better con- 

 trasts than a 45° prism with separate lens F, as in this case more 

 reflections must occur. 



A careful comparison was carried out. The result was that it 

 was not possible for us to trace any difference in the working of 

 the two kinds of prisms: the 45° prism is practically, so far as 

 contrasts are concerned, not at all inferior to the Le Chatelier prism. 



All comparative experiments were so made, by a special projec- 

 tion arrangement, that both prisms (or metal mirror) were exactly 

 in the same position, covering the half of the back lens of the 

 objective. 



It was found that for lower magnifications than with the apo- 

 chromatic f = 2 mm., the Le Chatelier prism has a decided advan- 

 tage over the 45° prism in so far as it is not necessary, for obtaining 

 a uniform illumination, to cover so much as half of the light area 

 with the Le Chatelier prism as with the 45° prism. Thus at lower 

 magnifications the aperture is better utilised with the Le Chatelier 

 prism. The arrangement mentioned above serves the purpose of 

 easily obtaining for each objective the proper position of the prism. 



6. Influence of the aperture. 



As already remarked, it has been pointed out by Dr. Rosenhain 

 and by one of the present authors that the definition of the image 

 at high magnification is considerably lessened when half of the 

 aperture of the objective is covered by a prism or by an opaque 



