Measuring Refractive Index and Dispersion of Glass. 409 



(2) It bus been shown that only a portion oi the ultimate 

 residual ionization in gases can be due to the collisions of 

 thermal agitation. 



(3) A formula has been devised for the number or collisions 

 per c.c. per second producing ionization which is in qualitative 

 agreement with experimental facts. 



In conclusion the author wishes to express his thanks 

 to Professor J. 0. McLennan, who suggested the problem, 

 and whose assistance and encouragement have been most 

 valuable. 



Physical Laboratory, 

 University of Toronto. 

 May "8, 19] G. 



XLV. ^4 New Method of Measuring tlie Refractive Index and 

 Dispersion of Glass in Lenticular or other forms, based 

 upon the ' ; Schlieren-methode" of Topler. By E. W. 

 Cheshire, B.A* 



THE methods most commonly employed tor the accurate 

 refractometric examination of optical glass for com- 

 mercial purposes involve special preparation of the specimen 

 under test. The Pulfrich refractometer, which is probably 

 used in optical workshops more frequently than any other 

 instrument, is designed to give an accuracy in the deter- 

 mination of 7* D of one unit in the fourth decimal place, and 

 in the partial dispersions of two or three units in the fifth 

 decimal place. As a preliminary to the examination on this 

 instrument the glass must be cut in the form of a right- 

 angled prism and, of the two faces forming the right angle, 

 at least one must be plane and well polished and the other, 

 in order to secure good definition through the telescope, 

 must meet it in a sharp knife-edge. The commercial 

 accuracy of determination in the optical constants specified 

 above is sufficient for practical designing purposes, and is, 

 in any case, greater than the accuracy with which the 

 refractive index of successive meltings of glass can be 

 reproduced on the large scale. 



It may happen, however, that one is confronted with the 

 problem of determining the optical constants of a piece of 

 glass in a form in which it is not possible to examine it on 

 the refractometer. The glass, for instance, may be a small 

 component lens forming part of a microscope objective and 



* Communicated by the Director of the National Physical Laboratory. 



