342 OPTICAL PROJECTION 



prism as a large one (if not too small to transmit the smallest 

 diameter of the pencil). The conveniences in manipulation 

 are so great, while the absorption of light is less, that so far 

 from a large analyser being an advantage, where such are 

 used the screen effects are inferior to those constantly ob- 

 tained by more modern and improved instruments. I add to 

 this, as Professor S. P. Thompson once remarked at the 

 British Association, that it is ' almost a sin ' under present 

 circumstances to throw any large prisms away as analysers, 

 when they are so badly wanted at the other end ! 



To cover the standard slides, a Nicol is required as 

 polariser not less than 1J inches clear diameter or field, and 

 2 inches is of course better still. Fig. 190 on the opposite 

 page is a section of a complete apparatus, as I arranged it for 

 my own use. It was constructed for me in the first place by 

 Mr. Darker, and with some slight alterations Messrs. Newton 

 & Co. have since made many instruments after the pattern 

 here shown, which has also been very generally adopted by 

 other opticians, as best fulfilling the requirements of the de- 

 monstrator at the smallest expense compatible with a Nicol 

 polariserj The references will explain the construction, and 

 the external appearance of the portion of the apparatus 

 in front of the polariser is the same as that of the front part 

 of the apparatus shown in fig. 194. The stage aperture 

 should be made large enough to take quite a thick wooden 

 frame, with another frame containing a quarter-wave plate or 

 an even film : sometimes the instrument is made with a 

 separate aperture or stage for a quarter- wave plate, between 

 the ordinary working stage and the polariser. The crystal 

 apparatus can all be removed or added in a few seconds, and 

 I prefer the analyser mounted with differently -sized collars at 

 the two ends, one of which fits the nozzle of F for ordinary 

 work, and the other of which, when pushed in, can be adjusted 

 to the proper position for crystal-ring projections to pass 

 through it. A quarter-wave plate can be inserted if necessary 



