568 The Bight Eon. Lord Bayleigh [March 29, 



surface is observed by the formation of Newton's rings with soda 

 light. After perhaps three-quarters of an hour, the depression corre- 

 sponds to half a band, i.e. amounts to \ A, and it appears to be 

 uniform over the whole surface exposed. Two pieces of plate glass, 

 3 inches square, and flat enough to come into fair contact all over, 

 were painted with wax in parallel stripes, and submitted to the acid 

 for such a time, previously ascertained, as would ensure an action 

 upon the exposed parts of \ A. After removal of the wax, the two 

 plates, crossed and pressed into contact so as to develop the colours, 

 say of the second order, exhibited a chess-board pattern. Where two 

 uncorroded, or where two corroded parts, are in contact, the colours 

 are nearly the same, but where a corroded and an uncorroded surface 

 overlap, a strongly contrasted colour is developed. The combination 

 lends itself to lantern projection, and the pattern upon the screen 

 [shown] is very beautiful, if proper precautions are taken to eliminate 

 the white light reflected from the first and fourth surfaces of the 

 plates. 



In illustration of the action of hydrofluoric acid, photographs* 

 were shown of interference bauds as formed by soda-light between 

 glass surfaces, one optically flat and the other ordinary plate, upon 

 which a drop of dilute acid had been allowed to stand (Fig. 2). 

 Truly plane surfaces would give bands straight, parallel, and equi- 

 distant. 



Hydrofluoric acid has been employed with some success to correct 

 ascertained errors in optical surfaces. But while improvements in 

 actual optical performance have been effected, the general appearance 

 of a surface so treated is unprepossessing. The development of latent 

 scratches has been described on a former occasion. \ 



A second obvious application of hydrofluoric acid has hitherto 

 been less successful. If a suitable stopping could be found by which 

 the deeper pits could be protected from the action, corrosion by acid 

 could be used in substitution for a large part of the usual process of 

 polishing. 



In connection with experiments of this sort, trial was made of the 

 action of the acid upon finely ground glass, such for example as is 

 used as a backing for stereoscopic transparencies, and very curious 

 results were observed. For this purpose the acid may conveniently 

 be used much stronger, say one part of commercial acid to 10 parts 

 of water, and the action may be prolonged for hours or days. The 

 general appearance of the glass after treatment is smoother and more 

 ti anslucent, but it is only under the microscope that the remarkable 

 changes which the surface has undergone become intelligible. Fig. 3 

 is from a photograph taken in the microscope, the focus being upon 

 the originally ground surface itself. The whole area is seen to be 

 divided into cells. These cells increase as the action progresses, the 



* The plates were sensitised in the laboratory with cyanine. 

 t Proc. Koy. Inst., March 1893. 



