THE PRESIDENTIAL ADDRESS il 



That certain .pigments are bleached by Hght is an observation 

 tliat must have obtruded itself from very early times — indeed, it is 

 one of the chief practical problems of dyeing to select pigments 

 which do not fade rapidly. If a part of the coloured surface is 

 protected by an opaque object — say a picture or a mirror hanging 

 over a coloured wallpaper — we get a silhouette of the protecting 

 object, which is in essence a photograph. 



Again, it is a matter of common observation that the human skin 

 is darkened by the prolonged action of the sun's light, and here 

 similarly we may get what is really a silhouette photograph of a 

 locket, or the like, which protects the skin locally. In this case we 

 are perhaps retracing the paths which Nature herself has taken : for 

 the evolution of the eye is regarded as having begun with the general 

 sensitiveness to light of the whole surface of the organism . 



The sensitivity of at all events the dark adapted eye depends 

 on the accumulation on the retinal rods of the pigment called the 

 visual purple, of which the most striking characteristic is its ready 

 bleaching by light. We can even partially ' fix ' the picture pro- 

 duced in this way on the retina of, for example, a frog by means of 

 alum solution. This brings home to us how clearly akin are the 

 processes in the retina to those in the photographic plate, even though 

 the complexity of the former has hitherto largely baffled investigation. 



There are then many indications in nature of substances sensitive 

 to light, and quite a considerable variety of them have from time 

 to time been used in practical photographic processes. But com- 

 pounds of silver, which formed the basis of the earliest processes, 

 have maintained the lead over all others. The history of photo- 

 graphy by means of silver salts cannot be considered a good example 

 of the triumph of the rational over the empirical. For instance, the 

 discovery of developers came about thus. The first workers, 

 Wedgewood and Davy (1802), had found that they got greater 

 sensitivity by spreading the silver salt on white leather instead of 

 paper. An early experimenter, the Rev. J. B. Reade (1837), was 

 anxious to repeat this experiment, and sacrificed a pair of white 

 kid gloves belonging to his wife for the purpose. When he wished 

 to sacrifice a second pair, the lady raised a not unnatural objection, 

 and he said, ' Then I will tan paper.' He treated paper with an 

 infusion of oak galls and found that this increased the sensitivity 

 greatly. It amounted to what we should call exposing and develop- 

 ing simultaneously. But, in using the method, it is easily observed 

 that darkening continues after exposure is over, and this leads to 

 beginning development after the exposure. This step was taken by 

 Fox Talbot a year or two afterwards. Instead of crude infusion 

 of galls he used gallic acid. Later pyrogallic acid was used instead 

 of gallic acid, and still survives. 



