264 
XIII.— AN APPLICATION OP POLARIZED LIGHT TO 
RESOLUTION WITH THE COMPOUND 
MICROSCOPE.* 
By Dan M. Stump, B.S., M.E., F.R.M.S. 
[Read October 19, 1921.) 
One Plate. 
Early in 1917 the author was interested in the resolution of a 
dry mounted Fasoldt thirteen band ruling, and had succeeded in 
faintly resolving the tenth band with 114,000 lines to the inch, by 
using overhead oblique illumination and an objective of 1*40 
numerical aperture. 
The following brief note was noticed on page 325 of the 1891 
edition of Carpenters “ The Microscope and its Revelations,” 
“ . . . when resolving striae with oblique light the effect is much 
strengthened by placing a Nicol analysing prism over the 
eye-piece. . . 
Applying this suggestion to the resolution of the Fasoldt 
ruling, when the prism was revolved to its position of maximum 
effect, the tenth band was found to be perfectly resolved, each line 
standing out clearly from the next, and by very careful adjust- 
ment, the lines of the eleventh band, with 127,000 lines to the 
inch, were faintly visible. 
This result was so very gratifying that experiments were made 
upon a number of objects having minute periodic structure, and it 
was found that in all cases where the prism was set so as to pass 
light vibrating in a plane parallel to the strise being shown, a 
clearer image of the striae was formed than when the prism was 
not used. The best results were usually obtained when the prism 
was placed in the path of the illuminating beam. 
Attempting to explain this phenomenon, it was experimentally 
found that the diffracted beam was polarized, the light vibrating 
only in a plane parallel to the rows of markings causing the 
diffraction. The theory was then advanced that the image of the 
structure near the eye-piece diaphragm was formed by the inter- 
ference of the polarized diffracted beam with only that part of the 
incident beam vibrating in this same plane ; and that the remain- 
ing components of the incident beam, not entering into the forma- 
* Extracts from a paper presented to the Armour Institute of Technology, 
Chicago, May 1, 1921. 
