Aldehyde. A substance differing from an alcohol by con-taining less hydrogen. The only aldehyde of special interest in connection with photography is the aldehyde of methylic alcohol which is sold as a 40 per cent, solution under the name Formalin, which see.
AlethoscopeI look at). A de-
vice of Signor Ponti, of Venice, in which a large single lens is used in looking at a transparency or an ordinary positive. A device not very different from the M Pantoscope," or " Lanterno-scope," now sold for looking at small transparencies or lantern slides. It was claimed for the alethoscope that it showed ordinary photographs stereoscopically - a claim which was disputed and even ridiculed. As bearing on the matter, the following may be quoted from The Amateur Photographer : " Physically it is impossible for one eye to see stereoscopically, and yet many persons by looking at a photograph with one eye can see it to a certain extent as stereoscopic. The most obvious explanation depends upon what a biologist would call a psychical effect as distinguished from physical. In looking at a natural scene first with both eyes and then with one eye, the effect of solidity very often fails to entirely disappear, as the mind recog-nises that the scene is solid, and this recognition prevents the physical image as cast by the lens of the eye being implicitly accepted. If, now, a person who fails to see a natural scene as completely flat when he views it with one eye, regards a photo-graph or picture with one eye, he sees that photograph just as he would see the natural scene if he closed one eye, and the association of ideas and perceptions gives it a stereoscopic effect to him."
Algraphy. A method of printing in which a plate of aluminium is the basis, the metal acting much in the same way as a lithographic stone. All the ordinary processes of photo-lithography can be adapted to Algraphy.
Alizarine. The characteristic colouring principle of madder, used in the form of an iron or alumina lake in preparing perma-nent pigments for " carbon " printing and collotype printing. For a method of using Alizarine to produce prints on calico, see Dyeing, Prints by.