An Application of Polarized Light. 
265 
tion of the image of the structure, naturally exerted an obscuring 
effect on the image, sufficient when this image was faint to 
entirely obliterate structure otherwise well within the resolving 
power of the objective. 
The function of the polarizing prism then was to remove all of 
this excess light not entering into the actual formation of the 
image of the structure, preventing this obscuring effect, and so 
allowing images of maximum clearness to be formed. 
A careful consideration of this analysis showed that by the use 
of a single beam of polarized light, the only improvement in any 
image that could be expected would be the sharpening of the 
striae in one direction only. No advantage could ’be gained with, 
an object having structure presenting the appearance of a crossed 
grating — that is, with two or more sets of superimposed parallel 
markings — since a single prism would at its best improve the 
definition in one direction only, and would probably destroy the 
images in other directions altogether. 
Since this type of crossed structure is very common in natural 
objects, and at the same time presents considerable difficulty in 
resolution, especially when the interval of the structure approaches 
the resolving power of the lens in use, it was considered very 
desirable that a method be found by which the advantages of the 
single polarized beam could be applied to the resolution of struc- 
ture of this latter type. 
The idea then evolved of illuminating the specimen with as 
many separate beams as there were structural elements, each beam 
being polarized in the proper plane to form a clear image of its 
particular element. The final image would then be a composite, 
formed by the blending of several independent images, each show- 
ing some particular element in the structure. The formation of 
an image in this manner is merely a modification of methods 
ordinarily used, tbe optical principles involved being identical. 
In June 1917, the accompanying photograph was made of the 
surface structure of the test diatom, Amphipleura pellucida, using 
two separate beams of oblique light polarized at 90° to each other. 
These beams were produced from two light sources, and directed 
through two Nicol prisms held in a proper position below the 
condenser of the microscope, by means of a cork mount inserted 
into the diaphragm carrier. With the microscope in a horizontal 
position, one of the beams was projected directly through one of 
the Nicol prisms, and the second beam was directed through the 
other Nicol prism by means of a small 90° reflecting prism placed 
directly below it. 
Another very convenient arrangement of the illuminating 
apparatus giving sufficient light for all visual work is shown on 
the accompanying sketch. A low voltage gas- filled lamp, furnished 
by automobile supply houses, may be placed between the mirror 
