xvi.] A PREDICTION. 211 



truth. We had been taking as the functions of light what were 

 really the functions of the bodies which received it, and it was, 

 therefore, quite easy to imagine, and one was justified in hoping, 

 that as the work went on we should find that what one par- 

 ticular kind of substance would do for the blue rays, another 

 particular kind of substance would do for the red rays and for 

 the green rays, and so on. 



My reference to this at the time was as follows : 



" I cannot but think, moreover, that when the light which the 

 spectroscope has already thrown upon molecular action shall be 

 better known and used as a basis for further inquiry, methods 

 of photography greatly exceeding the present one in rapidity, in 

 the less-refrangible portion of the spectrum, will be developed 

 and utilized in the research." 1 Fig. 79 will show how fully 

 this prophecy has been fulfilled by the results obtained by 

 Captain Abney within the last few years. 



In order to carry on this work a spectroscope of large dispersion 

 was necessary, and its observing telescope was replaced by a 

 camera of long focal length. In the instrument used at the 

 commencement of the research the whole solar spectrum from 

 beyond H to the red fell upon a five-inch plate. 



The laboratory in which the work was carried on had a 

 window nearly due (magnetic) south. Outside the window 

 level slate slabs were placed as supports for a heliostat. The 

 spectroscope was supported on a platform on rollers, the height 

 of the platform being such that the horizontal beam from the 

 heliostat was coincident with the axis of the collimator. This 

 general arrangement is shown in Fig. 80. 



In order to obtain photographs of the solar spectrum of the 

 very best kind, it was necessary to limit the beam passing 

 through the prisms 'to one of very small dimensions a 

 method previously employed with such admirable results by 

 Mr. Eutherfurd. 



1 Proc. Roy. Soc. No. 158, 1875. 



