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338 THE PHOTOGRAPHIC NEWS. [May 29, 1885. matters, it will be evident that in work of the nature of that in question a larger scale than this can seldom be used. “ If, however, it be desired in the case of purely architec tural surveys to work to a larger scale, it is only necessary to draw the circle to scale on the paper, and to make the construction from its periphery instead of from its centre.” The reason for keeping the vertical axis of the camera behind the optical centre of the lens is evidently to cause the two to coincide; would involve a most cumbersome construction of operation ; and would be a thing to be avoided unless very great advantages were to be gained I from it. This will be evident to anyone who will consider । how it would be necessary to construct a camera if it were desired that the screw pinching it to the tripod head should I be immediately below the lens, and at the same time that I the whole arrangement should be quite rigid. A PHOTOGRAPH BY MOONLIGHT. OUR illustration is a phototype block made by Mr. W. T. Wilkinson from an original photograph taken by Mr. J. B. Metcalf. Mr. J. B. Metcalf’s picture was taken by moonlight on the night of December - 13th, 1883, from the window of his residence in New York. The exposure extended from 6.30 to 11.30, p.m., and he used one of Eastman’s (American) special plates, Dallmeyer's 6} by 8} rapid rectilinear lens, full opening, and ferrous-oxalate (no bromide) developer. THE ISOCHROMATIC WET PROCESS WITHOUT THE INTERVENTION OF YELLOW GLASS. BY DR. H. W. VOGEL.* About a year ago I published in the Photographic News an account of my eosine collodion process for taking coloured subjects. In this process, as in all others pub lished afterwards, a yellow glass is necessary through which the subject ought to be taken, for reducing the action of the white and blue rays. Now it is said that Dr. Albert and Braun (Dornach) possess a secret isochro- matic process which does not want a yellow glass. This is an advantage, because the yellow glass increases the exposure necessary for taking a picture. Now it is easy to vary my published process in such a way that it works also without a yellow glass, though 1 confess that the proportion of brightness of the different dyes taken in this way is not so perfect as in photographs taken with azaline plates. During the last year I published a paper on the import ance of the silver eoeide for the isochromatic wet process. • Communicated by the Author, from a chapter of his new book: “The Photography of Coloured Subjects in the Right Proportion of their Brightness,” which book will be published in Germany.during the next few Amory and Abney have already shown that this substance is only sensitive for green and yellow rays of the spectrum, and I thus explain the favourable action of it in the wet | isochromatic process. This eoside is formed in the wet ' eosine collodion process, and can be easily observed when one submits a plate coated, with eosine-dyed collodion to the action of a nitrate of silver bath made acid with acetic acid. ! It is a well-recognised fact in photography that in many । cases two sensitive substances mixed together influence each other favourably. Iodide of silver or bromide of ; silver when used alone in the wet process do not possess sufficient sensitiveness for the shadows, but when mixed they are far more satisfactory. The same kind of thing holds good in the case of the spectrum. Iodide of silver by itself is, with a short exposure, only sensitive a little beyond G, while bromide of silver is sensitive to F ; but both together are sensitive as far as b or E. The case is similar with silver eoside. The formation of the required silver compound which not only gives developable images, but is also at the same time an optical and chemical sensitiser, is the cause of the exceptionally active action of eosine in the wet process. A notable circumstance is that the silver eoside plates lose very little of their beautiful red colour in the fixing bath, hence it would seem that the eoside of silver is only decomposed with difficulty by hyposulphite of soda. The following facts relative to the sensitiveness of films con taining eoside of silver were established by me in 1884:— 1. Eoside of silver by itself is sensitive for the yellowish- green and green rays of the spectrum, the action going