86 



DISPLACEMENT INTERFEROMETRY BY 



Two plates of glass were examined without difficulty, the absolute value 

 of n depending on the purity of the benzol. This appears in the last result 

 for the first plate. The screw micrometer has a much wider range than the 

 Billet compensator. It admits of the trial of thick plates. It does not require 

 standardization. On the other hand, however, it is not so sensitive. A com- 

 parison of the two shows, for instance, 



Screw 3.1 6.4 9.8 18.4 



Billet. .. .95.9 79.0 61.0 17.1 



10 3 cm. 

 scale-parts 



so that about 5 scale-parts go to one iQ- 3 cm. of the screw, both of which 

 can be read to tenths of these divisions. 



TABLE 7. Refraction of plates submerged in benzol. t= 47 10'. 

 Constants as in Table I . 



* Fresh benzol. 



63. Summary. The present method of using the self-adjusting inter- 

 ferometer has an advantage as compared with the older one (in which but 

 one of the interfering beams traversed the trough), inasmuch as the wall 

 of the plane-parallel trough and the liquid contained are of no influence on 

 the results. Such an effect would be added to both beams under the same 

 conditions. Moreover, by putting the glass-test objective into the beams suc- 

 cessively twice (2 Ax) the micrometer displacement is obtained and the null 

 position is of no consequence. 



On the other hand, there are distinct disadvantages, because the range of 

 displacement of the compensation micrometer (vacuum air compensator, 

 Billet wedge micrometer) are limited. The spectrum ellipses are rather too 

 large for convenience, therefore. One might add glass relay plates counter- 

 posed by air-paths to the Billet wedge ; but these (even if of optic plate) are 

 liable to modify the fringes. For the same reason the dispersion constants of 

 a single glass plate can not be easily found, a problem for which the older 

 apparatus was particularly well adapted. 



Some other liquid with a refraction between benzol and aniline (probably 



