PHOTOGRAPHY. 17 



had "been active. The length of the ultra-violet part of 

 the spectrum is equal in length to the whole of the 

 visible spectrum. Mr. Lockyer and Professor Stokes, I 

 think, have told you about these ultra-violet rays, and I am 

 not going to repeat what they have said. Instead of using the 

 electric light we may pass a very thin slice of sunlight 

 through a prism or a couple of prisms, and when we do so, 

 we find a spectrum traversed by black lines, which you have 

 already heard are due to the absorption of metallic and 

 other vapours. Those lines, as you are aware, occupy certain 

 fixed positions in the spectrum, and supposing that we get 

 photographic impressions bounded by any particular line, we 

 should know what part of the spectrum was effective. 



Before proceeding further I may show you that with the 

 ordinary silver salts employed the red light is inoperative to 

 produce a picture, whilst the blue light is perfectly capable 

 of so doing. I have here a dry plate prepared with 

 bromo-iodide of silver (I told you yesterday that we mixed 

 bromides with iodides in the collodion, and produced silver 

 bromo-iodide in the film by means of a solution of silver 

 nitrate), and I will expose, for a minute, one half of it behind 

 a negative to the red rays, and the other half for ten seconds 

 to the violet rays. You will see that in the first half of the 

 picture we shall get no results, and in the other half we 

 shall get an image. After it is developed I will throw it on 

 the screen. Here it is after the developer has been applied, 

 and you see the red light is incapable of impressing an image. 

 Before I came yesterday I photographed the spectrum on 

 different silver compounds, to show what rays are capable of 

 producing an image, and when you compare this photograph, 

 taken on the iodide salt, with the carefully coloured drawing 

 hanging on the wall, you will find what I told you was 

 correct about the silver iodide. [The photograph shown 

 on the screen.] The transparent line, which you see at 

 the extreme end of the image, agrees with the line E of the 

 solar spectrum, and when we turn the light on the diagram 

 of the spectrum you will see what position this line occupies 

 in the green. The spectrum goes along through the blue, 

 through the violet, and here we get two well-known lines, 

 called the H lines, also to be seen in the diagram, which 

 are very near the extreme end of the violet portion. You 

 can now see that the photographic action goes far beyond them, 



