APPARA TUS FOR SCIENTIFIC DEMONS TRA T1ON 171 



of rays from the lens is reduced to its smallest diameter at 

 the apparent crossing-point. 



A right-angled prism is ordinarily employed, but is not the 

 best form, as a large portion of the prism at the apex P is 

 quite outside the field of all inverted rays. By making the 

 prism with all this waste portion truncated, the same effective 

 field can be obtained with a much less massive piece of 

 optical glass, and the cost thereby much reduced. Another 

 method is to employ a greater angle than 90, and Zentmayei 

 adopted an angle of 126, which utilises all the field to the 

 apex. Such a large angle, however, loses appreciably more 

 light by reflection ; and, on the whole, I consider the best 

 results are obtained by a combination of the two methods, 

 adopting an angle of about 105 and truncating the prism 

 sufficiently to have no surplus glass. 



An erecting prism should not be used unnecessarily, as its 

 adjustment takes time, and there is considerable loss of 

 light. 



87. Optical Front. For general use in optical and some 

 other experiments, a special focussing front is desirable, of 

 construction some- 

 what different from 

 the ordinary one for 

 exhibiting slides. I 

 have found the ar- 

 rangement shown 



in fig. 94 very con- "!. 94.-o P ticai Front 



venient. AB is a 



tube or fitting which fits nicely into the lantern nozzle like the 

 usual one for diagrams. B o is a tube 3 in. diameter which 

 screws into a collar at B, and having a rectangular opening E 

 cut through both sides to serve as a slide-stage. The opening 

 should be 1^ inches by 2f inches, so as to receive easily slides 

 over 1 inch thick, and 2 J inches wide. These slides are pressed 

 to the back by a flange L, forced against the slide by a spiral 



