Effect of the Spectrum on the Haloid Salts of Silver. 249 



M On the Effect of the Spectrum on the Haloid Salts of Silver." 

 By Captain W. de W. Abney, C.B., R.E., D.C.L., F.R.S., 

 and G. S. Edwards, C.E. Received November 26, — Read 

 December 12, 1889. 



In 1881 one of us gave, in the 'Proceedings of the Royal Society,' a 

 paper with the same title as the above. Since then, however, he has 

 been able to work out a more exact means of measuring the effect of 

 the spectrum on these salts of silver, and it is our desire now to lay 

 the improved results before the Society. 



In January, 1887, one of us read a paper before the Photographic 

 Society of Great Britain, " On the mode of measuring Densities of 

 Photographic Deposits, with some remarks on Sensitometers," and in 

 it alluded to the possibility of measuring the relative sensitiveness of 

 a photographic plate to the different parts of the spectrum. The 

 plan there indicated, with some instrumental improvements, has 

 been employed in the present instance. 



The method employed consists in throwing an image of the photo- 

 graphed spectrum on a white screen and measuring the density of 

 the photograph at different points. As the spectrum of sun light 

 abounds in dark Fraunhofer lines, it was evident that the sun 

 would be a very inconvenient source of light by w T hich to form the 

 spectrum. It was also inconvenient on account of the variation 

 in intensity at different times of the year and day in its different 

 parts. After trials of various sources of light we came to the 

 conclusion that the most practical source to employ was the light 

 from gas, burnt in an Argand burner. A somewhat whiter 

 light would have been better, perhaps, since the ultra-violet rays 

 would have been stronger ; but it appeared that, taking all things 

 into consideration, the convenience of gas light more than counter- 

 balanced its disadvantages. We may mention that the crater of the 

 positive pole of the electric light was used in some instances ; but, as 

 certain minima of action on some of the salts of silver experimented 

 with lay at parts of the spectrum where bright carbon bands were to 

 be found, the main researches were carried out by the aid of gas light. 



The apparatus employed for photographing the spectrum was that 

 employed in the previous research already alluded to. The two 

 prisms employed were of medium dense white flint, each having an 

 angle of 62°. The collimating lens was of the same material, and the 

 photographic lens was a rapid rectilinear doublet by Dalmeyer, of 

 16-inch focal length. In some cases one of the lenses of the doublet 

 was dismounted, and the other used as a single lens, giving a focal 

 length of about 30 inches. An image of the gas flame was thrown 



