﻿Method for examining Optical Qualities of Glass Plates. 617 



(2) Using tbe instrument as it is * (or better with a 

 rather larger collimator and telescope and shorter air-path) , 

 the different parts of the strip may be compared with one 

 another and finally with the ends, whose u optical thickness " 

 <3an be separately referred to the standard. 



Unfortunately, the first method, which is the more efficient, 

 involves somewhat elaborate and substantial fittings for 

 adjusting and fixing the mirrors. 



Should it be found essential to figure the whole plate 

 before cutting it up, the great advantage of the present 

 instrument in having reference bands which are part of the 

 same optical system as those under examination will have to 

 be sacrificed. Burton's signal, mentioned above, placed 

 •on a travelling micrometer might be used, or another set 



Fig. 3. 



STANDARD 

 PLATE 



PLATE UNDER TEST 



of reference bands could be obtained by modifying 

 arrangement shown in fig. 3, so that part of the two be 

 pass clear of the mirrors A and B to be reflected into 



telescope by mirror C. 



* A geometric fitting for ensuring that the standard plate and s 

 always have tlio same orientation would have to bo provided. 



the 



ams 



the 



tops 



