This is still a secret with some in the profession. A limited number of workers have succeeded in bringing out good opals, and their modus operandi is kept from the many. Now this is a pity, when one considers the great charm attached to a good picture on opal, with pure whites and rich blacks, and in many localities the demand that might be created for them. Apart from their beauty, another charm attaches to opals--their absolute permanence; and this, it must be allowed, is no trifle. What, in fact, can be more painful to the worker who values his work, and sets store by it, than to feel it must ere long fade and pass into oblivion! A properly executed opal will no more fade than the glass pictures so common at one time, and which, wherever taken care of, are as perfect now as they were when first taken.

Now, excellent pictures are to be made on opals by means of emulsion; but I propose first taking the transfer method (mainly applicable to ground opal and canvas) as given above for pottery, since in practice it is found very ready, easy of manipulation, and safe. The details are much the same as above, and necessitate double transfer.

After the picture had been obtained on the plate (ordinary glass plate), and after thoroughly fixing, washing, and toning, the picture (and this, remember, is the case likewise with terra-cotta) then has to be loosened from its support, and this is done with a solution of sulphuric acid--one drachm to fifteen ounces of water--which is made to flow between the image and the glass, after which perfectly wash and mount. When the image is loosened a piece of tracing paper is put on the image, evened out, raised (assisted by some one else to hold the two opposite corners during the operation), and with the aid of the helper the picture is carefully centered, gently pressed out or down, and the transfer is so far effected. But what will happen, and does happen, in the case of vignettes, is impurity of the whites, when the picture becomes positively objectionable. Now the way to remedy this lies simply in the application, to the dirty-looking parts, of a solution of iodine dissolved in iodide of potassium to sherry color; after which, well wash and apply a weak solution of cyanide of potassium, and wash well again. This, by the way, is equally applicable to paper transfers; and it is to be remembered that the toning comes last of all.

It is a rather difficult matter to clean a ground opal which has been used two or three times, and acid must then be had recourse to (nitric acid is as good as any); but by transferring from the support on the ground surface, all stains are at once avoided.

On the flushed glass, or on the pot metal (unground), after well cleaning the surface it should be covered with a substratum of egg. Then the picture is taken direct, not transferred; that is, the plate is exposed direct in the camera, regularly proceeded with, and, when dried, varnished with a pale negative varnish, or with dead varnish if intended for chalk or water-color. This, when a good negative is used, gives a remarkably fine picture, not requiring a vestige of retouching, and having likewise the invaluable advantage of being perfectly durable if varnished with the negative varnish. Moreover, on that, effective pictures may be made in oil with simply tinting.

A gentleman, who has a right to be considered a good judge in all art matters, on looking at one of these pictures transferred on flushed glass, said it was one of the finest productions of photography. He urged that negatives ad rem should be taken most carefully, and that, like the picture I showed him, they should be full of half-tone and detail, and yet have plenty of vigor. They should, he said, be robust in the high lights, have perfectly clear glass in the few points of deep shadows, and thus have powerful relief. Moreover, the negatives should be retouched only by a competent hand, and care taken that the likeness shall be in no way altered, which is so frequently the case now.

If done as thus suggested there is no doubt that remarkably fine pictures are to be produced on opal, whether ground or not. Most artistic results are to be obtained, and, with proper care, absolute permanency. In this age of keen competition, all have to think of what may be really recommended to one's clientèle, and likely to meet with approbation from strangers and friends when the picture has once been delivered; and I candidly think that the opal, of all, is the picture most likely to meet with this general approbation.

I hope I have left it clearly to be understood that the class of opal picture to which I have chiefly alluded is one that remains untouched after the transfer--that is, absolutely unpainted upon. It is pure photography in every sense of the word, and the resultant picture one hardly to be surpassed in any way. I have rather laid a stress on this point, well knowing how pictures are at times irretrievably ruined by the barbarous hand of would-be artists, who by far exceed the true artists in number; and the hint on retouching should not be lost sight of, either, at a period when the tendency is to stereotype every one in marble-like texture, or rather lack of texture, as if the face were devoid of all fleshiness and as hard and rigid as cast-iron. It might be wise to weigh this point carefully, and act upon it, before the enlightened public have raised a cry against the pernicious practice and made photographers smart for their want of applying timely remedial measures to a decided evil.

On reading the above again, fearing lest any misconception should arise in the mind of the reader, I deem it expedient, to clearly state that for terra-cotta recourse is had to double transfer; that is, the picture first taken is lifted from the support on tracing paper, put in the right position on terra-cotta, and pressed down while wet with blotting-paper, left to dry, and is then so far ready.

Respecting the production of pictures by means of emulsion, ground opal being the best, the system I employ is as follows: After well cleaning the glass, coat it with emulsion (which had better not be too thick). When dry it is exposed and developed with the usual oxalate developer, to which a little bromide of potassium has been added. The remainder of the operations is as usual. Those varnished with dead varnish can be tinted and worked up with colored crayons or black lead pencil and make very pleasing pictures. It is needless to add that they are also to be finished in water-colors if thought preferable.--G. W. Martyn, in Br. Jour. Photo.