16 LECTURES TO SCIENCE TEACHERS. 



PHOTOGRAPHY. 



LECTURE II. 



WE left off with the development of the dry plate by the 

 method known as the alkaline process, and this morning 

 we must break into new ground. But yesterday after the 

 lecture a gentleman came to me, and said, "You have not 

 spoken about the latent image." Now what is called the 

 latent image is a thing I hesitate to recognise. I will admit 

 there is an invisible image, but . not what should strictly be 

 called a latent image. If we take 10,000 parts of oxide 

 of zinc, say, and mix thoroughly with it one part of lamp- 

 black, you will perceive no difference in tint between the 

 pure oxide of zinc and that contaminated with the lamp- 

 black ; but the lampblack is there nevertheless. So in the 

 same way when we expose for a short time a sensitive plate 

 to the action of light, a change has taken place in certain 

 molecules exactly in the way that would give us a visible 

 image by a longer exposure, the only difference between 

 the one and the other being in the quantity of molecules 

 affected. I think I need scarcely repeat the experiments 

 which showed the development of the invisible image. 

 Suffice it to say that the visible as well as the invisible 

 image, can attract silver from an unstable solution of silver 

 nitrate. 



But to-day we have to consider as to the light which is 

 most favourable for photographers, i.e. to what rays of light 

 these salts of silver are most sensitive. If you cause a slice of 

 white light to pass through a prism, it is separated into 

 its component rays, and we have what we all know as the- 

 spectrum. The spectrum of the light from the incandescent 

 carbons is now thrown on the screen, and if I were to place 

 a large piece of paper impregnated with a sensitive silver 

 salt such as the iodide in that spectrum and I have taken the 

 iodide with a set purpose we should notice, on the appli- 

 cation of a developing agent, that no change in its molecules 

 had been effected by the red and yellow rays, but that green, 

 blue, violet, and the invisible rays beyond the violet 



