OPTICAL GLASS FOR WAR NEEDS 113 



would practically disappear when the whole mass again reached 

 the same, that is to say, room temperature. This method of 

 annealing has been found satisfactory for small and ordinary- 

 sized blocks and effects a great saving in time. 



It is obvious that with such rigid requirements it becomes 

 necessary to develop adequate methods for testing. . The inter- 

 ference figure observed in the black field of a polariscope with 

 crossed prisms is customarily used as an indication of strain. 

 Dr. Wright of the Geophysical Laboratory devised a test based 

 on the assumption that because two rays of different index are 

 formed as a result of the strain, these seriously affect the image 

 formed. The path difference of two such rays is, therefore, 

 a measure of strain and this can be expressed in millionths of a 

 millimeter per centimeter of path traveled. In a well annealled 

 glass this path difference is 5, a fairly good glass 10, and that 

 which is barely usable 20 millionths of a millimeter. In 

 Government inspection the value 10 was ordinarily used, with 

 between 5 and 10 as the standard in special cases. 



The refractive index and dispersion are two physical con- 

 stants of utmost importance and successful glass-making means 

 turning out pots of the same glass within one in the third deci- 

 mal place in refractive index. Dispersion is of fundamental 

 importance when designing lenses to avoid aberrations, and is 

 expressed as the V value. This is the ratio of the refractive 

 index for the D or sodium line, minus one, to the difference 

 between the refractive indices for the F and C lines. 



Uniformity in the constants is very necessary, for of course 

 all grinding and polishing tools cannot be changed with every 

 batch of glass. Lead increases refractive index as well as 

 dispersion, and extends the blue end of the spectrum. Barium 

 raises the index, but does not relatively increase the total dis- 

 persion nor extend the blue end of the spectrum to the same 

 extent as does lead. Zinc is intermediate in its effect, while 

 calcium raises slightly both the index and the dispersion without 

 extending the blue. Boron cuts down the total dispersion. 



Freedorq from color is also, a prime consideration and because 



