13 THE PRESIDENTIAL ADDRESS 



The use of gelatine as a medium to contain the gilver halide was 

 a more obvious idea. But it was not so easy to foresee that the 

 sensitivity of silver salts would be much further increased when they 

 were held in this medium. For long this remained unexplained, 

 until it was noticed that some specimens of gelatine were much 

 more active than others. This was ultimately traced by S. E. 

 Sheppard to the presence of traces of mustard oil, a sulphur com- 

 pound, in the more active specimens. This, in turn, depends in 

 all probability on the pasturage on which the animals that afford the 

 gelatine have been fed. The quantity present is incredibly small, 

 comparable in quantity with the radium in pitchblende. 



The value to science as well as to daily life of the gelatine dry 

 plate or film can hardly be overestimated. Take, for instance, the 

 generalised principle of relativity, which attempts with considerable 

 success to reduce the main feature of the cosmical process to a geo- 

 metrical theory. The crucial test requires us to investigate the 

 gravitational bending of light, by photographing the field of stars 

 near the echpsed sun. For this purpose the gelatine dry plate has 

 been essential : and here, as we have seen, we get into complicated 

 questions of bio-chemistry. This is to my mind a beautiful example 

 of the interdependence of different branches of science and of the 

 disadvantages of undue specialisation (or should I say generalisa- 

 tion .''). We may attempt to reduce the cosmos to the dry bones 

 of a geometrical theory, but in testing the theory we are compelled 

 to have recourse again to the gelatine which we have discarded from 

 the dry bones ! 



To come back, however, to the development of the photographic 

 retina, as I may call it. As is well known, the eye has maximum 

 sensitivity to the yellow-green of the spectrum, but ordinary silver 

 salts are not sensitive in this region. Their maximum is in the blue 

 or violet, and ranges on through ultra-violet to the X-ray region. 

 It was not at all easy to extend it on the other side through green, 

 yellow and red to infra-red. The story of how this was ultimately 

 attained is one more example in the chapter of accidental clues 

 skilfully followed up which forms the history of this subject. 



In 1873, Dr. Hermann Vogel, of Berlin, noticed that certain 

 collodion plates of English manufacture, which he was using for 

 spectrum photography, recorded the green of the spectrum to which 

 the simple silver salts are practically insensitive. The plates had 

 been coated with a mixture which contained nitrate of uranium, 

 gum, gallic acid and a yellow colouring matter. What the purpose 

 of this coating was is not very obvious. It rather reminds one of 

 mediaeval medical prescriptions which made up in complexity what 

 they lacked in clear thinking. But Vogel concluded with true 

 scientific insight that it must owe the special property he had dis- 



