Science Lectures at South Kensington, by Capt. Abney [And Others].

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878. Excerpt: ... 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 lampblack, you will perceive no difference in tint between the pure oxide of zinc and that contaminated with the lampblack; 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 thespectrum. 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...