PHOTOGRAPHY. 



21 



shaken off by it when the swing of the wave is sufficiently- 

 inharmonious. It should be noted, however, that in these 

 loaded molecules the increased weight may be a dead weight, 

 as it were ; that is, the part shaken off may still be chlorine, 

 iodine, or bromine, if the compound contain any or all of 

 these halogens. I now throw on the screen a photograph of 

 the spectrum taken on loaded silver bromide. The bright 

 line in the centre is the extreme limit of visibility, the A 

 line ; on the left beyond the A line there are indications of 

 bands of lines, and when you come to examine microscopically 

 a good photograph, you can see that it is particularly rich in 

 lines, a great many of which are due to the absorption of 

 the atmosphere. 



FIG. 6. 



To show you that it is not only the solar spectrum that 

 can be photographed by this means, I have here the first 

 photograph taken by myself of the red end of a spectrum, 

 of a metal, namely, of calcium. This metal is particularly 

 rich in red rays, together with some rays beyond the extreme 

 limit of visibility. I may remark in passing that by the 

 process here adopted the point of maximum photographic in- 

 tensity in the spectrum is lowered towards the red. Those 

 two black lines on the left are the last which can be seen, 

 and the lines towards the right lie beyond the A line. Their 

 waves are of such a length that they cannot impress them- 

 selves on the nerves of the eye. 



I should like to expose a plate coated with this sensitive 

 salt, to show you that there is no illusion about photo- 

 graphing with red rays. I have here such a one which I will 

 expose to the red light only ; it will require perhaps a 

 minute's exposure, and I must have it developed as far as 

 possible in the dark, otherwise the image would be veiled. 

 [The developed image was then thrown on the screen.] 



Dr. Vogel, of Berlin, has experimented with bromide 

 plates prepared in the silver bath, which, after washing, he 

 nooded with various diluted dyes. He has found and often 



