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January 30, 1891.] THE PHOTOGRAPHIC NEWS. 81 want to try, and see that it is not just your own propor tions differently stated. Secondly, do not imagine that, by changing the proportion of bromide some minute frac tion of a grain, you are to notice a sudden and wonderful improvement in your work. A little longer exposure or a little longer in the developer will bring you just where you were again, perhaps quite unintentionally on your part. And, thirdly, recognise that if you know precisely when the picture which you are aiming at is before you, there is not much difficulty in bringing it there. As I stated in the early part of my paper, my plan is to develop a thin image by means of a weak developer of no special formula : and after this is well out I pour it off, rinse the plate, and apply a more concentrated solution. Whether under-exposed or over-exposed, this is equally the best treatment, and leaves the operation thoroughly under control. I have only two other points which I should like to mention. The first is that, after hundreds of experiments, I find none of the substitutes better than pyro. The second is, that in cases of under-exposure, if, after soaking the plate in developers, it be taken out before the solution begins to get brown, and washed gently under water, put into an empty dish and covered over with a sheet of glass to prevent evaporation, you will find as a rule a beautiful though thin image in a few hours, which will thicken up to any amount by a stronger developer applied then. TESTING THE SENSITIVENESS OF PHOTO- GRAPHIC PLATES. A long article appeared recntly in the Chemical News from the pen of Professor Victor Schumann on “ Determining the Sensitiveness of Photographic Plates by means of the Spectrograph.” The author begins by speaking of War- nerke’s sensitometer as an instrument of limited efficacy, but not entirely useless. In the case of orthochromatic plates, however, its advantages have, he says, become “ seriously sceptical,” even for practical working, apart from abstract scientific purposes, for no other light but blue is emitted by the phosphorescent plate. All sensitometers, says the author, have been super seded by the trustworthy spectrograph, by the aid of which more distinct information on the sensitiveness of photographic plates is obtained. Whoever desires to learn of their relations to the action of light rays of diffe rent colours should resort to the spectrograph. Heavy flint glass, like that of the Amici prisms, should, however, be entirely excluded from the instrument; these glasses are impenetrable to ultra-violet. Violet even, and not less so indigo and blue, are held back with much energy, while the luminous rays, yellow and red, are allowed to pass through them without any hindrance almost. Such prisms must naturally lead to erroneous sensitometer numbers. On account of partial absorption, yellow is reproduced with excessive force, and plates found to be highly sensitive for yellow and red in the spectroscope will fail to show the same properties with objective exposures. Lenses and prisms made of crown glass, or of light flint with less dispersive power, are preferable for ordinary purposes. With any camera, and without much trouble, a suitable spectrograph may be easily constructed. To obtain faultless spectrum photographs, a scrupulously correct instrument is not necessary. I have made the sharpest pictures with precision instruments as well as with apparatus but roughly built, and even pro fessional spectroscopists have made the most astounding discoveries with improvised instruments. To find the behaviour of plates towards any light source, a series of spectra, with regularly increasing time of ex posure, should be photographed, the plates dried, and be examined in regard to the time required to produce a developable impression by different colours. The differ ent sensitiveness will then be found to be reversely pro portioned to the time of exposure. Were we, for example, to inquire how the sensitiveness of a Vogel eoside of silver plate (manufactured by Prentz, of Munich) is proportioned to our own instrument, the first spectrum of the series to be photographed should be exposed for such a short time as not to produce a developable impression. Subsequent and longer exposure will then generally produce an image of extremely feeble character—a mere breath upon the plate. The thinner the picture of the spectrum, and tho clearer ground of the plates, the more reliable is the sensitometer number derived from the series of spectro grams made by increasing time of exposure. As long as sensitiveness alone is the object of inquiry, the intensity of the plate is of no importance whatever. It is totally wrong to determine numerical value of light-sensitive films by the density of the negative. Sensitiveness and intensity are two entirely different things, as every photographer knows who understands the use of the sensitometer. How little hold is offered by the intensity of the plate, when inquiring into its sensitiveness, is proved by the fact that the two never go hand in hand. Some plates resist strenuously the action of certain rays before decomposition of the light-sensitive components takes place ; but when their stability has yielded at all to the force of the developer, the intensity of the negative increases with much rapidity. Density builds up easily when the exposure has been longer than required for the production of the breath-like picture ; not proportionately, however, but progressively. This phenomenon occurs with Vogel’s eosine plates to an unusual extent. A hun dred times have I had opportunity to observe these properties when associated with Dr. Zettnow, of Berlin, and to whom we are indebted for the most important researches on erythrosine plates, to examine extensive series of orthochromatic plates. Other phenomena occurring and observed by studious investigators may bo well explained by this progressive increase of density, pre-eminently among them two peculiarities—the very changeable intensity of the maximum in yellow, and different yellow action even under strictly normal con ditions. Whoever wishes, with the foregoing explanation on hand, to determine the yellow and blue sensitiveness of erythrosine plates, will find that the former never reaches the height, numerically, stated by Dr. Vogel when the exposure is made at high altitude of the sun. Far from mid-day, or in winter time, the proportions of brightness in the sun spectrum are entirely changed. The general sensitiveness of the plate advances then more and more towards the red end of spectrum. When the sun stands low, at its rise and set, the erythrosine silver plate may triumph to a still greater extent, for then it is nearly ex clusively sensitive to yellow and red, but to no other rays. There will be a photographic exhibition at the Crystal Palace in the course of this year.
