ISOCIIROMATIC OR COLOUR-SENSITIVE PIIOTOGRAPFIY.
will soon become apparent, to even the most unobservant reader, that photography does not with even approximate accuracy reproduce the true gradations of light and shade, nor render in monochrome colours according to their proper value. Ordinary plates are almost insensitive to red, scarcely more so to orange and yellow, and reproduce most greens badly, at the same time being unduly sensitive to blue and violet. Chemists, recognising this shortcoming of photography, have long turned their attention to the pro ducing of plates which shall be more equally sensitive to the various colours of the spectrum, and the result of their investigations has been the introduction of what are known as isochromatic or orthochromatie plates. The terms are practically synonymous, and signify equal or true rendering of colour. It would be beyond the scope of a practical and elementary guide-book to go into the history of the process here, or to describe its nature, except in the briefest possible manner. Fortunately for the amateur, isochromatic plates are now an article of commerce, therefore it will not be necessary for the reader, from a practical point of view, to trouble himself with any details of their manufacture. They may be produced in two ways : first, by clipping, or bathing, ordinary plates in a solution of erythrosine, one of the very numerous coal-tar derivatives ; plates so prepared, however, will not retain their keeping qualities, and therefore must be used directly after being prepared. The alternative method is clue to M. Tailfer, a Frenchman, and is the subject of a patent ; and Mr. Edwards' isoehrornatic plates, produced under licence by this process, have acquired a well-deserved reputation for their uniform reliability and excellence. I have used isochromatic plates from the time of their intro duction, and for mores purposes I consider they are far superior to ordinary plates. Photographers generally, how ever, have until recently been slow to adopt them, or to recognise their advantages ; one probable reason for this I shall refer to later. So very much better are they than
ordinary plates that I have not the least hesitation in ex pressing the opinion that the day is not very far distant when colour-sensitive plates will be almost exclusively used ; and the fact that quite recently one of the largest firms of plate-makers* have considered it worth their while, as a business speculation, to purchase the right of making such plates from Messrs. Edwards, is strong evidence of the correctness of the assertion. Finality, however, in the preparation of colour-sensitive plates is very far from being reached, and several gentlemen who have devoted much attention to the investigation of the subject are still hard at work in this direction.
From a practical point of view isochromatic plates may be regarded as superior to ordinary plates for the following special purposes : copying oil or water-colour paintings and coloured diagrams, landscape work generally in which green or yellowish green is the prevailing colour, and more particularly when the foliage has assumed the brown and yellow tints of autumn ; they also render clouds better and more truthfully than do ordinary plates, while portraits taken upon them require less work from the retoucher's pencil.
The one drawback to the use of isochromatic plates, if such it can be said to be, lies in the fact that they can only be safely manipulated in light of a deep ruby colour, and to a disregard of this precaution a good many failures are readily attributable. It needs very little reflection to demonstrate that a plate which is capable of accurately rendering orange and yellow cannot safely be manipulated in a light of that hue. Ruby light only, and that of a proper character, should be employed. It is better not to expose the plates unnecessarily to even ruby light, and in filling the dark-slides it is well to get as far from the light as possible. During development a light wood or cardboard cover should be placed over the dish until the image is well out.