The chemistry of photography by Meldola Raphael 1849-

Author:Meldola Raphael, 1849- [from old catalog]
Language: eng
Format: epub
Publisher: London, New York, Macmillan and co., limited
Published: 1901-03-25T05:00:00+00:00

M L

if G

VwUft BUie

F D C B A.

Fig. s.—Visual Intensity of Colours of Solar Spectrum.

From such a diagram it is at once seen that the maximum of visual intensity is in the yellow, a little to the more refrangible side of the line D, It IS seen also that the visible spectrum extends from near the line A in the red to about ff in the extreme violet. In a similar way the distribution of heat might be represented by another curve, the maximum of which would in this case be in the infra-red. The same method is applicable to the representation

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of photographic intensity. The amount of decomposition undergone by a silver haloid in different parts of the spectrum is shown by the degree of darkening on long exposure, or by the density of the silver deposit with short exposure and development. It is only necessary to make the lengths of the ordinates (Fraunhofer lines) proportional to the degree of darkening, and then connect the abscissae by a curve as before. The curve then shows at a glance the distribution of photochemical efficiency in every part of the spectrum for the particular salt employed.

We are now, I venture to think, in a position to follow the results of recent work in spectrum photography. The action of the pure spectrum upon the silver haloids in various conditions and in different vehicles has been studied by many investigators, especially by J. W. Draper, Hunt, Herschel, and Becquerel, and most recently by Abney with improved appliances, and with greater care in eliminating the effects of impurities than has hitherto been bestowed upon this branch of work. I shall of course be unable to

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254 THE CHEMISTRY OF PHOTOGRAPHY.

do more than give you a general notion of the results obtained. The first question to be asked is whether the different haloids have the same maxima of sensitiveness, and this can be answered by experiment in a somewhat rough and ready way by allowing our spectrum to fall on three slips of paper coated with the respective haloids and, after a sufficient exposure, developing in the usual way [E]. The result shows that with the chloride the maximum is in the violet, just about or a little beyond the limit of the visible spectrum ; with the bromide the maximum is in the blue, and with the iodide also in the blue, but a trifle more towards the violet end. You will notice also that the bromide spectrum is longer than either of the others—that it extends more towards the red, a fact of which we shall see the practical significance hereafter.

The experiment shown is necessarily a hasty one, and gives only a crude idea of the local action of the spectrum upon the different haloids. In more exact work the greatest precautions have to be taken to insure trustworthy results

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PHOTOGRAPHED SPECTRA,

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—the effect due to every substance associated with the haloid, whether as an essential constituent of the film, or as an accidental impurity, must be studied individually before we arrive at the true photographic spectrum of the haloid.

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