Sensitive-strip Spectropolariscope. 165 



cylinder of glass or of this fluid would totally reflect the 

 extraordinary ray and transmit almost completely, and without 

 any displacement as is usually the case in nicols, the ordinary 

 ray. A second cylinder containing a narrow thin strip of the 

 ^spar would give us a bisected or trisected field with a 

 vanishing line. 



In cutting * the spar for both the first polarizer and for 

 the sensitive strip, two directions in the crystal have been 

 tried. In the first system the longest axis of the plate lies 



( . in a principal plane through the optic axis, making an angle 

 of 70° with it. The first plate was cut about 2 mm. thick 

 and in the form of an ellipse with its edges ground away so 

 as to fit within the cylindrical cell containing the liquid. 

 The dimensions of this ellipse were 44 mm. by 15 mm. over 

 one face, thus giving a clear circular aperture of 15 mm. 

 with an angle of incidence of 70°. The sensitive strip was 

 cut in the same plane but only 5 mm. by 44 mm., and rect- 

 angular in shape. The thickness was 0'15 mm. and its lateral 

 edges perfectly sharp and square with its faces. Experience 

 with plates of these dimensions and thicknesses show that 

 they may be safely reduced to 0*1 mm. or less in thickness. 

 In the second system the plates were all cut rectangular and 

 to the same dimensions as those of the first. Their direction 

 within the crystal, however, is different, their faces being 

 principal planes through the optic axis which is perpendicular 

 to the longest edge or length of the plate. More difficulty 

 was experienced in grinding these as they fractured more 

 readily and a thickness of 0*5 mm. was found practicable. 

 More material was also required for this system. 



In order to obtain total reflexion of the extreme red rays, 

 an angle of incidence of 65° is necessary. To provide for 

 total reflexion of a cone of rays, 6S° to 70° is required. The 

 angle of incidence adopted is 70°. This gives a length of 

 40 mm. for a cell of 15 mm. clear aperture. The cost of a 



,<; plate itself is about one-third that of a nicol of the same 

 aperture. The ellipse or first plate is mounted within a cell 

 of about twice the diameter of the aperture in order to 

 provide for sufficient fluid and effective diaphragming. There 

 are five of these diaphragms fixed to a concentric tube and 

 they are slightly slotted along a diagonal so as to admit and 

 retain the polarizing plate. This latter is cemented slightly 

 at its ends to the blackened tube with fish-glue (Le Page's), 

 mixed in four parts with two of glycerine and five of water. 

 Care is exercised in avoiding any strain as depolarization will 

 * The plates were cut and polished by Bernhard Hallo. Steglitz-Berlin. 



