Manchester Memoirs, Vol. Ix. (1916), No. 0. 9 



no absorption takes place. Fig. 1 shows an example of 

 this form of absorption. Dyes which have their chief 

 absorption at that point of the spectrum where their exact 

 complementary colour lies, and whose absorption on in- 

 creased exposure spreads out equally on both sides of this 

 centre give naturally the best results. 



Some dyes again have two or more absorption bands 

 as shown in Fig. 2 This is not of much account so long 

 as they are not too far apart, the centre of the absorption 

 is at the complementary point, and the bleaching occurs 

 equally in all the bands. As dyes do not always bleach in 

 all their absorption bands this leads to many anomalies. 

 Many blue dyes are peculiar in this respect. I was 

 astonished one day to find a blue dye which bleached 

 much more readily under its own colour than under the 

 complementary colour. This I found was due to the 

 action of the violet and ultraviolet rays where the dye 

 bleached rapidly, while there was little bleaching in the 

 absorption bands in the orange. The dye bleached practi- 

 cally as a yellow dye, and not according to Liesegang's 

 bleach-out proposition. Under a yellow colour the violet 

 and part of the ultra-violet rays were cut off, so the dye 

 bleached slower than under a blue colour which allowed 

 these rays to pass. Fig. 3 shows an example of a dye of 

 this description. 



Many dyes — and again, in particular, blue ones — had 

 other peculiarities. Some would recuperate their colour 

 partially in the dark, while others would bleach out but 

 have their colour restored on dipping in a weak acid 

 solution. These dyes were of course valueless. 



As I was not successful in finding three dyes which 

 fulfilled the necessary conditions sufficiently, I was led to 

 introduce a larger number of dyes into the emulsion, with 

 a view to the one equalizing the action of the other. In 



