334: PROCEEDINGS OF THE ACADEMY OF [DeC, 



POLARIZATION AND COLOR EFFECTS EXHIBITED BY CERTAIN DIATOMS. 



F. J. KEELEY. 



The June, 1917, issue of the Journal of the Royal Microscopical 

 Society, reprints a paper by E. M. Nelson, describing polarizing 

 effects shoAAai by certain diatoms, in which the suggestion is made 

 that ''it may be that silex deposited by animals does not exhibit 

 polariscope effects, while that deposited by plants, such as diatoms, 

 will do so." Hence a satisfactory explanation of tljis effect seems 

 desirable. 



When plane polarized light impinges on the surface of a transparent 

 isotropic substance so oriented as to be neither parallel nor normal 

 to the plane of polarization, each ray is, to a greater or less extent, 

 resolved into two polarized rays, vibrating parallel and normal to 

 the plane of incidence, transmitted and reflected respectively. If the 

 angle of incidence happens to correspond to the angle of maximum 

 polarization of the reflecting substance, the effect will be almost 

 complete, but only partial at other angles. Such reflected and 

 refracted rays, therefore, vibrate in planes different from that of the 

 original polarized light and would pass through an analyzer placed 

 at right angle to the polarizer. 



In using a polarizing microscope arranged to illuminate the object 

 with parallel rays and an objective of small aperture, there is little 

 chance of such reflected or refracted rays entering into the formation 

 of the image, and isotropic substances appear dark between crossed 

 prisms no matter what their shape, but the arrangement described 

 by Mr. Nelson includes a condenser above the polarizer and an 

 objective of fairly wide aperture, .5 N. A., under which conditions, 

 rays reflected up to the maximum polarizing angle or their refracted 

 components, may be transmitted through the objective, and the 

 corresponding structural elements of the object will appear bright 

 on a dark field, or modify the field color if a selenite film is interposed. 

 The structure of diatoms and other minute silicious organisms is 

 now sufficiently well understood to render it apparent that the 

 innumerable surfaces capable of reflecting light, amply account for 

 the polarizing effects noted, and, when illuminated under the de- 

 scribed conditions, all diatoms, radiolaria, sponge spicules, powdered 



