494 



Profs. Liveing and Dewar 



[June 20, 



H 74 to H 98, 1755, 1761, 1781; mean, 1766; differences, - 

 0-011, - 0-005, + 0-015. 



It will be seen that, even with prisms such as these, by taking the 

 mean of different determinations, the uncertainty can hardly be as 

 great as one half per cent. 



Extremely small prisms are quite sufficient for the determination of 

 the ratio of the dispersions of the glasses by the above method. It 

 may, however, happen that an optician cannot afford to remove even 

 so small a piece of glass from a disk intended for an objective, and has 

 not a specimen of glass on the identity of which with the glass of his 

 disk he can thoroughly rely. In such a case it is necessary to de- 

 termine the optical constants of the disk by means of facets cut on the 

 disk itself. A heavy and costly disk cannot be treated like a small 

 prism, and mounted on a small graduated instrument in the manner 

 I have supposed a small prism treated. To compare the ratio of ihe 

 dispersions of two such disks, one of crown glass and the other of 

 flint, the most convenient way would seem to be to leave the disk 

 fixed, let the light pass through it first, and then achromatize it by a 

 small prism of very low flint glass, mounted on a small graduated in- 

 strument in the manner already explained. The dispersions of the disks 

 would be compared with each other by comparing them in succession 

 with the same intermediate prism. 



This, however, demands an additional determination beyond what 

 was required in the other process, since the prism through which the 

 light first passes is not the same in the two cases. The element which 

 best lends itself to measurement is the angle of incidence on the first 

 surface. The most convenient mode of measuring this must depend on 

 the general disposition of the apparatus adopted to take the measure- 

 ments for the angle of the disk and the deviation of some one line, 

 which must be made in any case ; it is accordingly best left to the 

 choice of the observer. 



XL " On the Reversal of the Lines of Metallic Vapours." By G. 

 D. Liveing, M.A., Professor of Chemistry, and J. Dewar, 

 M.A., F.R.S., Jacksonian Professor, University of Cambridge. 

 No. III. Received June 19, 1878. 



In our last communication to the Royal Society we described cer- 

 tain absorption lines, which we had observed to be produced by the 

 vapour of magnesium in the presence of hydrogen, and certain other 

 lines which were observed when potassium, and others when sodium, 

 was present, in addition to magnesium and hydrogen. These lines 

 correspond to no known emission lines of those elements ; but, inas- 

 much as they appeared to be regularly produced by the mixtures 



