350 BELL SYSTEM TECHNICAL JOURNAL 



in the instrument using the immersion fluid. When, in such instruments, a 

 polarizing filter is so oriented as to cut oflF one or both short prism-lines, these 

 lines may be restored to view by rotating the test tube in the holder (upon 

 continuous rotation one line and then the other will disappear). With 

 vertical conoscopes the mirror may be removed for bottom illumination. 

 When monochromatic light is used all three lines are the same color as the il- 

 luminant. When using non-monochromatic or white light, the long, slit 

 line is the color of the source, while the short lines develop into two, separate 

 spectra. In this case, that portion of the spectrum is used for alignment 

 which is most predominantly used in the immersion instrument. The 

 sensitivity of the refractoscope, when approaching perfect match, is about 

 An — .001. This sensitivity is of course attained when one adjusts the 

 fluid to match the quartz, by addition of the proper high or low-index com- 

 ponent. When the fluid does not match there is a less accurate measure of 

 mismatch, but this measure is still good for determining the degrees of fluid 

 adjustment to be made. 



The principle of the refractoscope may be even more simply applied to 

 measuring the fluid to quartz mismatch, by making use of the inspectoscope 

 and a basal section of quartz, using the 120° prism formed between two good, 

 clear, adjacent natural faces. The section is placed base down in the tank 

 at the far side from the mercury' lamp, and so positioned that a vertical slit 

 diaphragm, placed on the other side of the tank, may be viewed through the 

 two prism faces. With the polarizing filters removed, the two images of the 

 slit as seen through the prism do not (in general) align with the slit as seen 

 above the prism. The image farthest from the prism vertex is the one that 

 should be aligned with the slit, for perfect match of refractive indices. The 

 necessity of removing the polarizing filters can be obviated by tilting the 

 prism and slit about the line of sight, preferably at 45° from the vertical (or 

 this might have been obviated, if the polarizing screens had been out with 

 their plane of polarization at 45° to the vertical). 



Finally, experience indicates that the importance of keeping the immersion 

 fluid clear and clean is not generally realized. As noted above, contamina- 

 tion not only gives bad scattering of the projection beam, but also 

 depolarizes the polarized light. A perforated plate raised from the bottom 

 of the tank is an aid in keeping the settled dirt from being recirculated 

 again. More effective is the provision of simple, easy means for draining, 

 filtering and refilling the tank. One or more thicknesses of chamois makes a 

 good filter, provided the chamois is occasionally washed out with a solvent. 



4.8 Photographic Study of Interior Defects 



The original inspectoscope of Fig. 4.5 was used at the Hawthorne plant 

 of Western Electric Company, in obtaining the accompanying photographs. 



