144 Mr. J. A. Tomkius on the Direct 



Summary. 



Further experiments with the mass spectrograph on the 

 elements Se, Te, I, Sb, and Sn are described. 



Of these only I gives definite results. It is simple and of 

 atomic weight 127. 



The work on xenon is repeated with better material and 

 results in the establishment of reliable values 129, 131, 132, 

 134, 136, for its five principal isotopes and the discovery of 

 two possible additional ones 128 and 130. 



Negative mass spectra of chlorine are described which 

 give valuable confirmatory evidence on the constitution of 

 that element already indicated as probable. 



Cavendish Laboratorv, 

 March, 1921. 



XII. On the Direct Determination of the Powers of Lenses. 

 By J. A. Tomkins, A.R.C.S., Technical College, Brad- 

 ford *. 



IN ophthalmic practice, as is w r ell known, lenses are usually 

 specified in terms of their pow T ers rather than of their 

 focal lengths. The power of a lens may be determined in 

 one of the following ways: — (a) From its focal length, as 

 measured by various methods, (b) by means of the sphero- 

 meter or lens measure, (c) by neutralization with a lens of 

 known power selected from a trial case — all more or less 

 indirect methods, (a) is simple and accurate and need 

 involve but little calculation ; (b) depends on a knowledge 

 of the refractive index of the glass. The lens measure, as 

 used in the optical trade, is a modification of, but less 

 accurate than, the spherometer. It is graduated for a 

 given refractive index, usually about 1"52, this being the 

 average refractive index of the glass used in ophthalmic 

 lenses ; (c) is the method employed in ophthalmic practice, 

 and it is recognized as the quickest and most convenient 

 method of checking the powers of lenses. 



But, by this method, in the case of strong lenses, there is 

 (1) the difficulty of getting complete neutralization over the 

 whole field, and (2) the fact that, owing to the non-coincidence 

 of the principal point?, the powers of the neutralizing lenses 

 are not equal. The condition for neutralization is the 

 numerical equality of adjacent vertex powers, the adjacent 

 vertices being in contact. 



* Communicated bv the Author. 



