The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Hygrometer                                        Indiarubber Solution
Sufficient for half-plate. It is especially recommended for chloride plates, bromide and Alpha papers.
Hygrometer. a measure). An
instrument for measuring the moisture of the atmosphere, and sometimes used by carbon printers. That known as a wet and dry bulb thermometer is convenient; but a long strip of un-sensitised carbon tissue, hanging up in the apartment, gives a sufficient indication to the expert.
Hypo. An abbreviation of Sodium Hyposulphite (q.v.).
Image. In optics a simulation of an object - real when it can actually be received on a screen, as a camera image ; virtual when the eye sees it as in the prolongation of bent rays entering the eye.
Image, Latent. The action of light upon the sensitive salts of silver has always been a question with scientific photo-graphers, and it would be impossible within reasonable limits to give all the arguments pro and con. For some considerable time past, and even now, a few photographers hold that the action of light upon the sensitive salts is to set up a vibratory motion, which the developer takes advantage of, reducing these vibrating molecules to a metallic state. This theory, however, is generally considered erroneous. It is now generally believed that sub-haloid salts are formed by the action of light. Carey Lea has formed these by purely chemical reactions, and he calls them photo-salts, and they would seem to be compounds of a lower haloid salt, with the normal haloid in varying proportions. In all cases there seems to be an evolution of the haloid element. It seems unlikely that all the molecules of haloid salt are reduced; on the contrary, but a very minute portion.
Confirmation of the dissociation of the molecule of bromide of silver by light has been given by the researches of Guntz, who has been able to prepare the sub-haloid salts of silver.
Imogen Developer. See Development and Developers.
Indiarubber Solution. This is used for edging plates or as a substratum for the collodion process, and also for mounting gelatino-chloride prints. Pure unvulcanised indiarubber (sold as masticated rubber or "cut sheet") may be dissolved in benzole,