22 LECTURES TO SCIENCE TEACHEES. 



correctly, that the film becomes sensitive to the action of those 

 rays which the dyes will absorb. Blue, however, according to 

 his theory, should absorb all the red rays, and thus be chiefly 

 sensitive to the red and yellow. Experiment does not always 

 bear him out in this. I am inclined to think that by the 

 use of these dyes he forms a real compound with silver (for 

 however well you may wash your plates you cannot eliminate 

 all free nitrate of silver), and thus loads his molecules. To 

 explain Vogel's reasoning I will throw the spectrum on the 

 screen, and place a glass cell containing cosine dissolved in 

 alcohol before it ; you see there is an absorption in the green, 

 whilst the fluid allows the rays immediately above and below 

 it to pass. 



You must bear in mind that where absorption occurs, there 

 work of some description must be performed, for the diminution 

 of the amplitude of a wave denotes that energy has been 

 expended. 



If we faintly stain a film of silver bromine with eosine, so 

 that the dye absorbs the green rays through its whole thickness, 

 and carries, as it were, the light to every individual molecule 

 in its path, the rays not so rapidly absorbed should not 

 cause such a rapid chemical change. By increasing the 

 strength of the dye on the washed plate it is possible to 

 prevent any change in the portion exposed to the green 

 rays, as they are absorbed before they get to the bromide, 

 but if at the same time we increase the silver in the film to 

 such an extent as to allow the whole of the dye to form 

 a compound with it, the effect is still more marked. By 

 this procedure I have been enabled to photograph below 

 the "a" line, with the weighted molecule of bromide. 

 I throw on the^ screen a spectrum taken on a plate 

 slightly stained with eosine (see Fig. 5), and I will ask you to 

 compare it with the absorption spectrum of the dye and also 

 with the spectra impressed on the other silver compounds. 

 As I said before, I believe that the results obtained with the 

 dyes by Vogel are due to their combination with the residue 

 of silver nitrate left in the film, and what seems to tend to 

 confirm this view is that if the film contain excess of soluble 

 bromide, no results are to be obtained. 



Dr. Vogel's researches do not absolutely point to obtain- 

 ing a method of prolonging the photographic spectrum, 

 yet they tend towards it. You must not accept my explana- 



