So "N n RAYS 



or whistles, or if a knife, or a slightly bent stick, 

 or the clenched fist, etc., be brought near to the 

 cardboard, all the spots will become distinct 

 and more luminous ; the nebula resolves itself. 

 When the rays are suppressed, the screen 

 resumes its former aspect. 



(2) To obtain large, uniformly luminous 

 screens, the process is similar to the one 

 adopted for painting in Indian ink; a coating 

 of the mixture of sulphide and collodium, made 

 very thin by the addition of ether, is spread 

 out as uniformly as possible with a water-colour 

 brush. When this layer is dry, a second is 

 applied, and so on until the screen appears 

 uniformly luminous. The thinner the coatings, 

 and the greater their number, the better the 

 result. 



(3) To measure the refractive indices and 

 wave-lengths, I use very narrow slits filled 

 with calcium sulphide. Two rectangular plates 

 of aluminium are placed side by side on a small 

 board, so that their edges are in contact. A 

 little of the metal was previously filed off one 

 plate, so that when the plates were in position, 

 a slit was formed between them 2 cms. long and 



