SOME attention is now being given to this subject, which, although somewhat old, is still very interesting; and is worthy of being better understood than it is at present.

It would be both unnecessary and operose for me to enter into the history of this branch of photographic printing; it is-sufficient to say that at one time opal photographs were positively uncared for. Since then; however, the photographic fashions, like all others, have undergone singular changes, and here in 1864 we find this subject absorbing a, considerable 'share of attention.

In THE BRITISH JOURNAL OF PHOTOGRAPHY of 25th November there is a very able leading article on the "Toovytype," and the whole process is lucidly explained in detail. In that article mention is made of some specimens on opal glass by Mr. Helsby, and I will now concisely explain the methods of producing them.

A negative is taken in the first place. There is then procured a long box, which is, in fact, the camera; one half slides within the other, and at one end a hole is cut, against which .the negative is placed, and outside the negative is a piece of ground glass; this serves to diffuse the light before passing through the negative. A large gas flame is used as a source of light; and if a reflector be placed so as to throw all the light through the negative, the exposure will be so much shorter. At the other end of the camera is the dark slide containing the sensitive plate of opal glass: of this more hereafter.. Midway between the plate and the negative is a wooden slide carrying the lens, which needs to be of such a focal length as will suit the box. Matters, then, stand thus: The light from the gas (or daylight may be employed) is reflected through the ground glass and the negative; the lens then takes up the image and throws it upon the sensitive plate. If a vignetted print is required the vignette may be placed either between the negative and the lens or between the lens and the plate--the latter I think preferable; this, however, rests with the taste of the operator. The sensitive plate is the most important matter in the process. The glass employed has only recently been manufactured, and is called "patent plate opal." The mode of its preparation is as follows:--The ordinary opal glass is taken, and the flashed or opal side is ground all over as is done with plate glass; this ensures perfect "flatness," and freedom from the "lumpy" condition mentioned in the editorial article referred to. This ground opal glass is then taken and polished by machinery, and the result is an article perhaps as beautiful as anything of the land yet made. Photographers are mainly indebted to Mr. J.A. Forrest, of Liverpool, for its introduction. To bring it to perfection Mr. Forrest has expended much thought and labour, but his exertions have been crowned with complete success.

A piece of this glass is selected and coated as usual with collodion, the preference being given to one that works clean, even if not as quick as another sample. The plate is then sensitised in a forty-grain bath, and is ready for placing in the camera. The exposure varies, with the light employed: it may range from a few seconds up to ten or fifteen minutes; but a, few experiments will decide this point. The development is accomplished either with iron or pyrogallic acid, and must contain a sufficiency of restraining acid to keep the pictures clean, without, however, erring on the side of harshness. Much care is requisite in developing, for the opal is of so pure a nature that the slightest stain or deposit thrown down under the developer will inevitably show itself against the white surface beneath, and completely mar the effect. The plate is cleared of iodide in the usual manner.

Mr. Helsby has made the following method of mounting the subject of registration under letters patent. He uses a case similar to those employed when positives on glass were common, and an oval opening is made through the back of this case. He then takes a vignette glass made in either pale blue or rose-coloured glass, and upon this he places the opal positive, and over the whole a covering glass, binding the edges of the three glasses with adhesive paper. The combined glasses are then pushed into the case, and on holding it up to the light is seen the vignetted portrait, shading off into a blue or rose-colour; and in this consists the whole of the patent.

Another method is to mount the pictures without the tinted vignette, in which case they are positives by reflection. Mr. Helsby claims that the pictures with the tinted vignette for viewing by transmitted light. are also adapted for viewing by reflection; but this, I think, is -a mistake, for .if we develope a picture far enough to have sufficient. depth of tone for a transmitted positive, we find invariably that the deposit has commenced on the clean portions, thus obliterating the details of the image when seen by reflected light. Pictures mounted without the vignette are not amongst the claims of Mr. Helsby's patent.

Perhaps a better way than either of the above methods is to print a positive for reflected light on a piece of opal glass of an elliptical form. When this is developed and fixed, a covering glass of exactly the same size, and with the edges bevilled and polished, is taken, and with a drop of Canada balsam is cemented upon the positive with the aid of a little heat from a warm iron plate, in the manner that has over and ever again been described in THE BRITISH JOURNAL OF PHOTOGRAPHY for cementing achromatic lenses together. When the balsam has set, the photograph is placed in a case closely resembling a lady's bracelet-case, and it is then complete.

About eighteen months since, Messrs. Harvey, Reynolds, and Fowler, of Leeds, introduced into this country a process called "photodiaphanie." Paper is coated with -a substance soluble in water---gum or gelatine, for instance-and when dry is coated with a thick film of albumen. The paper is then sensitised on a strong bath of nitrate of silver, and printed in the ordinary way under a negative till the shadows are deeply bronzed. The print is then removed, cut clear on the edges, washed, toned, and immersed. in the hyposulphite of soda fixing bath. The albumen film now commences to blister all over, and when the print is fixed it is removed to clean water. The film now floats off the paper, and can be taken up, when it is put into several changes of water to free it from the hyposulphite of soda. A piece of opal glass is then placed under the floating film in a large vessel of water, and very carefully raised out with the film lying upon it and the superfluous water having been drained off the plate, it is then set up to dry. When dry, I found that the film contracted very much, and consequently separated from the glass. I. tried acetic acid, as recommended, but without the slightest advantage, and ultimately I covered the plate with gum spread on with a, brush and allowed to dry; when this is introduced under the film in the water, the gum has just time to soften and thus to bind the film firmly to the glass. The; film when dry is varnished like a negative, and is then capable of being roughly handled. The films can be applied to curved surfaces as well as flat ones. This process is very similar to the "Toovytype"--with this exception, that the picture is printed on paper, which allows of its being examined during the printing, and the film is afterwards transferred to glass. The prepared paper can be had from the firm named.

Liverpool, Nov. 29th, 1864. GEORGE F. WILLIAMS.