150 Transactions of the Society. 



expected, even if ordinary cover-glasses are employed instead of 

 more optically perfect scales worked to uniform thickness. 



If such an analyser be attached to the pocket lens or hand 

 microscope, which can be held in suitable inclined position, it is not 

 necessary to have anything more elaborate than one or more glass 

 slips laid on a dead black background as a reflecting polarizer. 

 The proper angle of reflection can easily be found by trial, but it 

 is rather convenient to make a set-square cut at this angle. This 

 will be a triangle in which two sides are at right angles and their 

 ratio is equal to the index of refraction, say 1 • 5 or 3 to 2 roughly. 

 When I was young I should have been very glad to make a 

 polariscope before I bought one, but I was deterred by the fact 

 that the book I had gave the polarizing angle in degrees and 

 minutes, whereas my protractor was only graduated in degrees ! 



For a dissecting microscope with a horizontal stage two reflect- 

 ing beams are required in order to throw the light upwards. It will 

 be found quite satisfactory to lay one frame horizontally and the 

 other one may be at 60° instead of the more accurate polarizing 

 angle of about 57°. Eays which are 3° on either side of the middle 

 ray will be reflected at 57° and 63° or 63° and 57°, and will then 

 be perfectly polarized at one or other of the frames, giving more 

 uniform polarization of the illuminating cone. 



By using several plates of glass in a polarising frame the 

 intensity of the reflected polarized beam is increased, but about 

 four plates are sufficient, and not much is gained by using a greater 

 number. On the other hand, the degree of polarization of the 

 reflected beam depends largely on having the frame backed with a 

 surface which is sufficiently absorbent of the transmitted light, 

 since if any of this is reflected from the backing it will be 

 completely transmitted back through the plates. 



In using a pile of plates as an analyser the degree of polariza- 

 tion depends on the completeness with which the light polarizable 

 by reflection is eliminated from the transmitted beam, and therefore 

 a larger number of plates is needed. But the proportion of 

 polarized to unpolarized transmitted light increases as the angle of 

 incidence is increased. Very fair polariscopic effects can be 

 obtained with as few as eight or ten cover-glasses if these be 

 indeed at an angle somewhat in excess of the " polarizing angle." 

 But it is better to use a large number, say from one to two dozen, 

 and the older treatises on natural philosophy contain tables showing 

 the number of plates sufficient to obtain practically complete 

 polarization at different angles of incidence, as found in Brewster's 

 experiments. If the angle of incidence falls short of the '• polariz- 

 ing angle " in any part of the field of view, that portion will hardly 

 show any polariscopic eftects unless a considerable number of 

 plates are used. A more uniform field of view could, of course, be 

 obtained by the use of two piles of plates turned in opposite 



