INTENSIFICATION 445. Choice at Method of Intensification. The method which, unfortunately, is generally em ployed (bleaching the image in mercuric chloride and blackening with ammonia) has contributed not a little to the discredit of intensification ; it destroys the details of the shadows and blocks up the high-lights ; in addition, a negative intensified by this method is very unstable, the image gradually fading without the possibility of renewing it by any method whatever. For tunately, it is very far from true to say that all methods of intensification merit the reprobation which is rightly applied to that just cited.
The principal advantage of intensification is that it permits the contrast of a negative to be increased after it has been examined under better conditions than by the non-actinic light of the dark-room. Properly-conducted intensi fication may be made to give the same results as would have been obtained by prolonging development ; it can even give better results, for example, when it is feared that, after a certain stage of development, fog may develop more rapidly than the image itself.
To be effective and certain, a method of intensification should increase proportionally the different densities of a negative, or at least it should not depart too far from this propor tionality. For this reason, one should mistrust intensifiers which yield negatives which are " too clean " ; the extreme clearness of the intensified negative shows that the fog has not been intensified and consequently the shadow tones also have not been increased, in spite of the fact that it is precisely these parts of the image which generally are in greatest need of intensification. The use of such an intensifier should therefore be confined solely to black and white subjects, that is to say, to copies of pen-and-ink drawings or similar originals.
It is essential that the intensified image should be no more unstable than the original negative. It is advantageous, lastly, if the intensified image can, if necessary, be further intensified or reduced at pleasure, if the optimum condition has been passed.
It may be added that the eye is a very bad. judge of the effectiveness of a method of in tensification, and that visual methods of photo metry fail when the image is not quite neutral (L. P. Clem, 1912). Thus, for example, the application to a photographic negative of one of the treatments, which will be described later under the name of "sulphide toning," gives an image which is less vigorous according to visual examination, but the effective contrast is augmented for printing purposes (A. H. Nietz and K. Huse, 1918).
We recommend the photographer who wishes to determine for himself the effectiveness of an intensifier to intensify half of a spare negative, and to print separately on the same paper the best possible print of each half, comparison being made on the prints.
446. In spite of the great variety in the methods of intensification, there are fairly narrow limits in the choice of a method giving the best result in a given case.
The amateur photographer will, as a rule, prefer methods by which intensification is brought about in a single operation (mercuric iodide intensifier), which makes control easier, or those methods which do not require the use of poisonous materials (chromium intensifier).
The physicist will prefer, in certain cases, the only method yielding exact proportionality between densities before and after intensification together with perfect stability of the image (negative bleached with mercuric chloride and then blackened with ferrous oxalate).
The maker of black and white reproductions has often no interest, indeed sometimes just the opposite, in preserving the tones of the nega especially when these tones are only due to slight spots or to inequalities of illumination of the original, and he is therefore only concerned with the increase of contrast ; the negative, bleached in mercuric chloride, is blackened by ammonia if only moderate intensification is required and if the negative need not be pre served ; or in silver cyanide, if considerable intensification is sought.