232 smith: polarized skylight and the microscope 



in all four of these types, and in this respect there is little to 

 choose between them; what slight preference there may be for 

 one type or the other will depend on the sky facing which one 

 adopts — or often must adopt ; on the time of day when the micro- 

 scope is most used; and on the importance of sky polarization as 

 a factor at noonday. F. E. Wright, 2 in a recent paper, has 

 concluded that for Washington, D. C, where "at noon time there 

 is always an abundance of light from a clear sky" (so that the 

 polarized scattered light may be disregarded) , and for a northern 

 facing, and a use of the microscope at any hour of the day, 

 "there is a slight advantage in having the plane of vibration 

 parallel to the vertical cross-hair." 



(4) A suitable compensator introduced below the polarizer of 

 the microscope, and capable of independent rotation about the 

 axis of the instrument, may effect practically complete correction 

 of the polarization of the skylight and at the same time give a 

 whiter and more favorable light. A simple type of such compen- 

 sator is a thin, parallel-faced plate of some transparent, bire- 

 fringent material — as quartz or muscovite — cut so as to give, 

 theoretically, with monochromatic light of 518 nn wave length 

 (the value which yields, with the petrographic microscope, results 

 most nearly in accord with the conditions of ordinary white light), 

 a phasal- difference of a half wave length between the entering 

 and emerging rays of light. Practically, it is cut so as to give 

 with ordinary light and between crossed nicols pure white of the 

 first order as an interference color. This compensator or half- 

 wave plate, mounted in a movable ring, should be free to rotate 

 about the axis of the microscope through an angle of not less 

 than 90°, and in use should be so turned that, theoretically, 

 its planes of vibration, for light with normal incidence, bisect 

 the angles between the planes of vibration of the reflected polar- 

 ized skylight and the polarizer of the microscope (see fig. 2) ; 

 practically, so as to obtain the maximum illumination. If the 

 compensator is tested between the nicols of a microscope — the 

 most severe test which can be applied — there is no observable 



2 Journ. Wash. Acad. Sci., 5: 641-644. 1915. 



