I will give you my own experience. I mentioned at the outset that seven or eight years ago I had tried Kennet's pellicle and failed, but got one or two results which I retained as curiosities till only a month or two ago; but up to that time I cannot say they had faded in the least, and I have here a specimen made three years ago, which I have purposely subjected to very severe treatment. It has been exposed without any protection to the light and damp and all the other noxious influences of a Glasgow atmosphere, and although certainly tarnished, I think you will find that it has not faded; the whites are dirty, but the blacks have lost nothing of their original strength. I here show you the picture referred to, a 12 by 10 enlargement on artist's canvas, and may here state, in short, that my whole experience of argentic enlargements leads me to the conclusion that, setting aside every other quality, they are the most permanent pictures that have ever been produced. Chromotypes and other carbon pictures have been called permanent, but their permanence depends upon the nature of the pigment employed, and associated with the chromated gelatine in which they are produced, most of pigments used, and all of the prettiest ones, being unable to withstand the bleaching action of the light for more than a few weeks. Carbon pictures are therefore only permanent according to the degree in which the coloring matter employed is capable of resisting the decolorizing action of light. But there is no pigment in an argentic print, nothing but the silver reduced by the developer after the action of light; and that has been shown by, I think, Captain Abney, to be of a very stable and not easily decomposed nature; while if the pictures are passed through a solution of alum after washing and fixing, the gelatine also is so acted upon as to be rendered in a great degree impervious to the action of damp, and the pictures are then somewhat similar to carbon pictures without carbon.
I may now say a few words on the defects and failures sometimes met with in working this process; and first in regard to the yellowing of the whites. I hear frequent complaints of this want of purity in the whites, especially in vignetted enlargements, and I believe that this almost always arises from one or other of the two following causes:
First. An excess of the ferrous salt in the ferrous oxalate developer; and when this is the case, the yellow compound salt is more in suspension than solution, and in the course of development it is deposited upon, and at the same time formed in, the gelatinous film.
The proportions of saturated solution of oxalate to saturated solution of iron, to form the oxalate of iron developer, that has been recommended by the highest and almost only scientific authority on the subject--Dr. Eder--are from 4 to 6 parts of potassic oxalate to 1 part of ferrous sulphate.
Now while these proportions may be the best for the development of a negative, they are not, according to my experience, the best for gelatine bromide positive enlargements; I find, indeed, that potassic oxalate should not have more than one-eighth of the ferrous sulphate solution added to it, otherwise it will not hold in proper solution for any length of time the compound salt formed when the two are mixed.
The other cause is the fixing bath. This, for opals and vignetted enlargements especially, should always be fresh and pretty strong, so that the picture will clear rapidly before any deposit has time to take place, as it will be observed that very shortly after even one iron developed print has been fixed in it a deposit of some kind begins to take place, so that although it may be used a number of times for fixing prints that are meant to be colored afterward it is best to take a small quantity of fresh hypo for every enlargement meant to be finished in black and white. The proportions I use are 8 ounces to the pint of water. Almost the only other complaints I now hear are traceable to over-exposure or lack of intelligent cleanliness in the handling of the paper. The operator, after having been dabbling for some time in hypo, or pyro, or silver solution, gives his hands a wipe on the focusing cloth, and straightway sets about making an enlargement, ending up by blessing the manufacturer who sent him paper full of black stains and smears. Argentic paper is capable of yielding excellent enlargements, but it must be intelligently exposed, intelligently developed, and cleanly and carefully handled.
THE MANUFACTURE AND CHARACTERISTICS OF PHOTOGRAPHIC LENSES.
At a recent meeting of the London and Provincial Photographic Association Mr. J. Traill Taylor, formerly of New York, commenced his lecture by referring to the functions of lenses, and by describing the method by which the necessary curves were computed in order to obtain a definite focal length. The varieties of optical glass were next discussed, and specimens (both in the rough and partly shaped state) were handed round for examination. The defects frequently met with in glass, such as striæ and tears, were then treated upon; specimens of lenses defective from this cause were submitted to inspection, and the mode of searching for such flaws described. Tools for grinding and polishing lenses of various curvatures were exhibited, together with a collection of glass disks obtained from the factory of Messrs. Ross & Co., and in various stages of manufacture--from the first rough slab to the surface of highest polish. Details of polishing and edging were gone into, and a series of the various grades of emery used in the processes was shown. The lecturer then, by means of diagrams which he placed upon the blackboard, showed the forms of various makes of photographic lenses, and explained the influence of particular constructions in producing certain results; positive and negative spherical aberration, and the manner in which they are made to balance each other, was also described by the aid of diagrams, as was also chromatic aberration. He next spoke of the question of optical center of lenses, and said that that was not, as had been hitherto generally supposed, the true place from which to measure the focus of a lens or combination. This place was a point very near the optical center, and was known as the "Gauss" point, from the name of the eminent German mathematician who had investigated and made known its properties, the knowledge of which was of the greatest importance in the construction of lenses. A diagram was drawn to show the manner of ascertaining the two Gauss points of a bi-convex lens, and a sheet exhibited in which the various kinds of lenses with their optical centers and Gauss points were shown. For this drawing he (Mr. Taylor) said he was indebted to Dr. Hugo Schroeder, now with the firm of Ross & Co. The lecturer congratulated the newly-proposed member of the Society, Mr. John Stuart, for his enterprise in securing for this country a man of such profound acquirements. The subject of distortion was next treated of, and the manner in which the idea of a non distorting doublet could be evolved from a single bi-convex lens by division into two plano-convex lenses with a central diaphragm was shown. The influence of density of glass was illustrated by a description of the doublet of Steinheil, the parent of the large family of rapid doublets now known under various names. The effect of thickness of lenses was shown by a diagram of the ingenious method of Mr. F. Wenham, who had long ago by this means corrected spherical aberration in microscopic objective. The construction of portrait lenses was next gone into, the influence of the negative element of the back lens being especially noted. A method was then referred to of making a rapid portrait lens cover a very large angle by pivoting at its optical center and traversing the plate in the manner of the pantoscopic camera. The lecturer concluded by requesting a careful examination of the valuable exhibits upon the table, kindly lent for the occasion by Messrs. Ross & Co.