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682 THE PHOTOGRAPHIC NEWS. [October 2, 1891. precautions are necessary here, but because the appear ance of the plate during development is different to that to •which he is accustomed. Let us remember that the alteration caused by the access of light to the film has affected the side next the glass, and not the side which lies upwards in the developing dish; so that the chemicals have to permeate a layer of gelatine which has hardly been affected at all by the exposure before they can get at the altered silver, and make the image visible. There is thus unusual delay in the image “ coming up,” and when development is actually far advanced, the operator is apt to think that it has hardly begun, for he sees it through a semi-opaque film of unaltered gelatine. He must therefore be careful to hold the plate often up to the red light, for it is only in this way that he can judge of the growing density of the deposit. Perhaps it would be well to develop all such plates in a dish with a glass bottom, through which progress can be easily seen without removal of the plate. If the photographer is accustomed to transparency making, he may prefer, to the method which we have recommended, the plan of making a positive on glass from the original negative, and copying that positive, with its film turned away from the camera, in order to get the reversal. This plan entails far more trouble than the other, and the negative ultimately produced is generally not nearly so good ; for, as everyone knows, in this tiresome process of getting positive from negative and negative from positive again, there is always a loss of pluck and a flattening of the image which is most destructive of its beauty. One can hardly say how and what it has lost; but there is a distinct deteriora tion all the same, and of course the fault is not remedied in the ultimate photo-mechanical print for which the reversed negative was undertaken. We have not said anything about stripping films from the glass, for in the case of dry plates this is generally a risky proceeding. If anyone wishes to try that method, let him by all means experiment with some spoiled plates of the same make before he plays any tricks with a valued negative. Many excellent methods of doing this work have been published, and we need here only refer to that fact. The best results from stripping are gained from employing the wet collodion process, for the film in this case is so tough, and free from any tendency to contract or expand, that it can be floated off one glass and attached to another without much difficulty, provided that the operator has a steady hand and clever fingers. Of course such a film ought to be coated with india-rubber solution and plain collodion before it is removed ; but the manipula tions are all described in the text-books, and need not be repeated here. It is doubtful, too, whether any dry plate worker will care to plunge into the many difficulties which must assail a tyro with the silver bath. Richmond Cambka Club.—On the 25th ult. the subject discussed was the eikonogen and hydrokinone developer. All present who had used the combined developer testified to its merits in case of snap-shots and under-exposed plates, SPHERICAL ABERRATION AND THE FOCAL PLANE. BY W. E. DEBENHAM. In an article on “Spherical Aberration” in your issue of the 18th ult., Mr. T. R. Dallmeyer speaks of the “absurdity ” of placing the focus midway in the locus of aberration, as done by Mr. W. K. Burton. He further gives, as a positive statement, his own view that the apex of the axial cone “ is the position of greatest concentration theoretically and practically, and isthe best focus, unmistak able to the practised eye.” It will, I think, not be difficult to prove, even to those who have not made a special study of photographic optics, that Mr. Burton is right beyond question in taking the position of best focus, as was done by the late Mr. Thos. Grubb, somewhere between that of the axial and the marginal rays; and therefore, that if there is any “ absurdity ” in putting forward a mistaken view on the subject, Mr. Dallmeyer must be content to accept the position in which he would place his adversary. If, as Mr. Dallmeyer says, the best focus is at the apex of the axial cone, then, as that position is fixed, and does not vary, however large may be the opening used, the change of diaphragm from large to small or vice versa would not, in the case of a lens having spherical aberration, affect the position of the focus any more than it does with a perfectly corrected aplanatic lens. Those who have worked with lenses having marked spherical aberration know very well that the insertion of a diaphragm does alter the position of the focal plane, focus being lengthened and brought more nearly to the place of focus of the axial ray in proportion as a smaller diaphragm is used, whilst with a larger aperture, or one sufficient to show more decidedly the existence of the spherical aberration, the focus becomes shorter; being, in short, a position,as stated by Mr. Burton, which is intermediate between the foci of the marginal and central rays. The fact of the shifting of the focal plane by the inser tion of a diaphragm in a lens having spherical aberration, is one so well known to photographers who practised when lenses of that character were more in use than at the pre sent time, that it seems strange that Mr. Dallmeyer should make a statement involving the negation of that fact. The American optician, Zentmayer, in the “ sixties,” con structed a lens the use of which depended upon the exist ence of this change of the position of the focus. The lens was made of one kind of glass only, and was not achromatised, in spite of which facts it was issued as working true to focus. This was accomplished by direct ing the operator to focus with the large aperture, and then, before exposure, to insert a small diaphragm of a definite relation of aperture. By this means the focus of the lens was lengthened just as much as was required to cor rect the chromatic aberration. If the focus had been as Mr. Dallmeyer’s proposition requires, always at the apex of the axial cone, the insertion of the stop would have made no difference to its position, and the lens would not have worked to focus. The reference to the practice of astronomers with regard to spherical aberration is quite beside the question. It has been already pointed out that, with an intensely luminous point, such as a star against a dark ground, there may remain light enough to form a visible image when the marginal rays are dispersed and only a small portion of the objective at the centre is actually forming the attenuated image, whilst with subjects such as photographers have to deal with the case is quite different.