=Isinglass= or fish glue, although, as before stated, a finer quality of glutin than the ordinary gelatine of commerce, is, on account of its high price and varying quality, comparatively little used. The addition of this substance also softens the gelatine layer. On the other hand, it possesses certain properties which render its use at times desirable; for instance, it does not dry so completely as gelatine, the layer prepared with it consequently gives cleaner and more even proofs, a point particularly noticeable in the earlier impressions. Such prints will also exhibit greater brilliancy, and this is of especial consequence in work produced from photographs from nature, in which class of work the half-tones are too often wanting. Some operators take even equal parts of isinglass and gelatine, but such layers must be very thin, and dried quickly. Thick layers are liable to crack or fly, and in use to refuse to take the ink, which must for such film be used in greater quantity and thinner than when gelatine alone is employed. A solution of isinglass is not so readily prepared as one of gelatine. It is necessary to allow it to soak in cold water for from twelve to twenty hours, and the temperature then gradually raised to nearly boiling point, at which heat it should be maintained for some time longer, and then filtered through flannel into the gelatine solution. In printing from these plates, particularly when the proportion of isinglass exceeds one-third the amount of gelatine, there is a disagreeable tendency of the printing paper to adhere in the high lights—that is, in those parts of the print layer which are represented in the negative by entire opacity—as in the sky of a landscape or a background which has been “stopped out” on the negative. On other portions of the plate where the light has more fully acted—during the printing in the printing frame—this troublesome defect will not be observed. The fault is more noticeable when the dull enamel paper is used, while a hard, well-sized letter paper presents the other extreme. Further details of printing papers will be found in another chapter. In the introductory chapter of this work the photo-chemical qualifications of the bichromates of ammonium and potassium with regard to collotype are considered.