﻿Intensification of Glass Diffraction Gratings. 585 



Ethylene dibromide gives a brilliant Swan spectrum in 

 which the groups a, 7, 8, and e were observed. Ethylene 

 dibromide melts at 0° C. The specimen employed was frozen 

 and allowed to melt slowly and drain, the last half to melt 

 being taken for the experiment, and this process repeated 

 several times. After standing in contact with zinc chloride 

 the liquid was distilled over into a discharge-tube connected 

 with an air-pump, and boiled in the tube before observing 

 the spectrum. The lines of the spectrum were quite sharp 

 when the pressure in the tube was small, but became less 

 sharp when the air was readmitted. 



Aniline and pyridine give the Swan spectrum brilliantly. 



In liquid naphthalene the spark gives the Swan spectrum 

 but the blackening is extremely rapid. The same result is 

 obtained with benzol. We have repeated an experiment 

 described by one of us (Phil. Mag. [4] xlviii. p. 456). Benzol 

 was distilled over sodium into a vacuum-tube containing bright 

 sodium at one end, and at the other a fragment of potassium 

 chlorate wrapped up in platinum-foil. This was connected 

 to the pump, the benzol made to boil, and the tube sealed off 

 when exhausted. This tube gave the spectrum formerly 

 described as C (2), consisting of the following " edges " : — 



6079-0 

 5610-3 

 5198-3 

 4835-4 

 4511-0 

 4393-? 

 4123-4 



On heating the sodium this spectrum disappeared giving 

 place to the Swan spectrum, but on heating the chlorate so as 

 to evolve a little oxygen the original spectrum reappeared. 



LXVIII. The Intensification of Glass Diffraction Gratings 

 and the Diffraction Process of Colour Photography. By 

 R. W. Wood * 



IN the course of some experiments made last year by my 

 assistant Mr. H. E. Ives on the diffraction process of 

 colour photography, much difficulty was found in ruling on 

 glass large gratings of sufficient brilliancy. Diamond points 

 could be found without trouble which gave very brilliant 

 rulings, but thev almost invariably broke down after ruling 

 an inch or so of the surface. Gratings which are to be 

 * Communicated bv the Author. 



