It is a remarkable and well-known fact that the chloride, iodide, and bromide of silver--called "sensitive salts" in photography--are not susceptible (at least only slowly) to change when exposed to the yellow, orange, and red rays. The longer wave lengths of the spectrum, as you know, form, with violet, indigo, blue, and green, white light. The diagram on the wall shows this dispersion and separation of the primitive colors. These--the yellow, orange, and red-- are called technically "non actinic" rays, and the others in their order become more actinic until the ultra violet is reached. The action of white light, or rays, excluding yellow, orange, and red, has the effect of converting silver chloride into a sub-chloride; it drives off one equivalent of chlorine. Thus, silver chloride, Ag_2Cl_2=Ag_2Cl+Cl. When water is present the water is decomposed. Hydrochloric acid, HCl, hypochlorous acid, HClO is formed. The iodide of silver in like manner is changed into a sub-iodide; but with water hydriodic acid is formed unless an iodine absorbent be present--then into hypoiodic acid. The silver bromide undergoes a similar change. When with light alone, a sub-bromide, Ag_2Br_2=Ag_2Br+Br, and with water hypobromous acid. It is important to bear this in mind, as one or other, and frequently both iodide and bromide of silver, is the sensitive salt requisite or used in producing the invisible image. The theory regarding these sensitive salts of silver is that, being very unstable, _i. e._, ready to undergo a molecular change, the undulations produced in the ether, which pervades all space, and the potential action or moving power of light is sufficient to disturb their normal chemical composition; it liberates some of the chlorine, iodine, or bromine, as the case may be. This action, of course, applies to light from any source--the sun, electricity, or the brighter hydrocarbons, also flame from gas or candle, whether it comes direct as rays of white light or is reflected from an object and conducted through a lens as a distinct image upon the screen of a camera. I have no time to speak on the subject of lenses, only just to mention that they are, or ought to be, achromatic, so as to transmit white light and of perfect definition, and the amount of light passed through should be as much as possible consistent with a sharp image--at least when rapid exposure is attempted.