smith: polarized skylight and the microscope 231 



Fig. 1. Diagram to illustrate the variation in the polarization of scattered 

 right from different parts of the sky. The short lines perpendicular to the lines 

 representing the sun's rays are the traces of planes of vibration of harmonic 

 motions which give rise to scattered light . A-B marks the approximate maximum 

 limits of the sun's diffraction glow, the circle of intense illumination close to the 

 sun, within which the effects of scattering are more or less masked and polariza- 

 tion is at a minimum. 



(3) The microscope may be rotated on its base until the plane 

 of vibration of the reflected polarized skylight coincides with 

 that of the lower nicol of the instrument. This is a simple 

 expedient, applicable at all times, and when the observer can 

 move with the microscope it is entirely satisfactory. When he 

 cannot, however, it necessitates his becoming familiar with the 

 use of the micropscope and its accessories in varied positions, 

 and even with such familiarity it is likely to lead to some 

 confusion. 



In this connection it may be noted that the orientation of 

 the plane, of vibration in the polarizer of the microscope may 

 have some effect in increasing or decreasing the difficulties with 

 polarized skylight. The plane of vibration of the lower nicol is 

 differently oriented in different instruments, even of the same 

 make. In the common type of the Bausch and Lomb petro- 

 graphic microscope, for example, this plane of vibration is square 

 with the instrument, in some cases running from front to rear, in 

 others from right to left. In the "Larson Model" the plane of 

 vibration is diagonal, sometimes in one direction, sometimes in 

 the other. The difficulties with polarized skylight are met with 



