18-i F. E. Wright — Tranxmixsion of Light through 



ends was first used, but was soon discarded because of the 

 effect of its oblique surfaces on the plane of polarization of the 

 transmitted waves. For exact work in extinction angles, the 

 ordinary type of nicol prism is much inferior to the G-lan- 

 Thompson type with square ends. Having thus crossed the 

 nicols accurately, the crosshairs of the ocular were adjusted by 

 using the ocular and Bertrand lens as a microscope and focus- 

 ing on a mounted anhydrite cleavage plate through which par- 

 allel sun's rays were passed centrally. Here again the sun's 

 raj's are so intense that the position of total extinction of the 

 anhydrite plate can readily be fixed within 1' of arc. By 

 means of the anhydrite plate which extinguishes parallel with 

 its cleavage edges, the principal sections of the nicols and the 

 ocular crosshairs were brought to coincidence. The universal 

 stage was then attached to the microscope stage and its hori- 

 zontal axis of rotation brought to coincidence with the hori- 

 zontal crosshair of the ocular, by use of the lines engraved on 

 the glass disk of the universal stage. This glass disk, together 

 with its supporting ring, was then removed and in its place a 

 second ring of precisely the same dimensions substituted, on 

 which a strip of thin glass plate was cemented, and to which 

 in turn one corner of the crystal plate was cemented, the glass 

 plate serving merely as a support for the crystal plate whose 

 major part was left free and exposed on both sides to air. 

 The surface of the crystal plate was then brought to approxi- 

 mate parallelism w T ith the horizontal circle of the universal 

 stage ; it was adjusted to exact parallelism by viewing, through 

 a mounted telescope, the image of a distant light source as 

 reflected from the surface of the crystal plate. The horizontal 

 circle H, of the universal stage 1 was then rotated and the crys- 

 tal plate tilted and turned by means of the horizontal circle H 3 

 and the vertical circle V„ until the reflected image remained 

 stationary on rotation of H 2 . The circles on the universal 

 stage could be read to 5' by means of the vernier, while on the 

 microscope stage the vernier intervals were 3'. In neither 

 case, however, were the lines on the circles and verniers suffi- 

 ciently fine to insure greater accuracy in reading than the 3' or 

 W intervals on the vernier. In actual work each position of 

 total extinction was determined 10 times and the average 

 taken. On sharp extinctions it was found that the different 

 settings were usually within 10' of the average. 



In the earliest preparations measured, the positions of total 

 extinction were determined by use of the bi-quartz wedge 

 plate, 2 but the fact that, in making the observations with this 

 plate, it was necessary to use the objective and ocular, the 

 glass surfaces of which in turn influence the plane of polar- 



1 See page 186 below. 2 This Journal (4), xxvi, 391, 1908. 



