230 smith: polarized skylight and the microscope 



stances which give the bluest skies. Hence it is that the polari- 

 zation of the skylight and the resulting loss of light in the micro- 

 scope are most evident on the brightest days, with the clearest, 

 bluest skies, and from those parts of the sky which are the 

 deepest blue ; moreover it is especially noticeable in the more arid 

 parts of the country and at considerable altitudes. 



The polarized skylight, reflected from the mirror of the micro- 

 scope, enters the lower nicol vibrating in a plane which may 

 or may not coincide with the plane of vibration of the nicol. 

 If the two coincide, there is of course no loss of light. When 

 they do not, however, there is always more or less loss, some- 

 times amounting to one-half or more of the total illumination 

 received by the microscope. 



Any remedy for the difficulty must involve either some change 

 in the illumination, or else a shifting of the microscope or a 

 modification of its optical parts. Among the possible devices 

 which would reduce or prevent the loss of light are the following: 



(1) Artificial light may be used as the source of illumination, 

 or a translucent screen may be interposed between the microscope 

 and direct sunlight. A cloudy or foggy sky has the same effect 

 as an interposed screen, the light from clouds being non-polarized. 



(2) The microscope may be so placed that the light will come 

 from a favorable part of the sky. As already noted, the degree 

 of polarization of the skylight varies in different parts of the sky, 

 being greatest in those portions from which lines to the sun 

 and the microscope make an angle approximating 90° with each 

 other, and decreasing rapidly on either side of this zone, more 

 especially with approach toward the sun (fig. 1). It also de- 

 creases close to the horizon, on account of the large number of 

 reflecting particles present in this portion of the sky. The 

 horizon belt, however, is generally too narrow and often too low 

 to be of much practical use; while near the sun the intensity of 

 the light becomes too great. Thus, while the selection of favor- 

 able parts of the sky may somewhat decrease the polarization 

 effects, it is not always a satisfactory remedy, especially when 

 the choice is restricted to those parts visible from a single 

 window. 



