'flu ° beginner will avoid spoiling a number of plates if he makes systematic tests to ascertain the correct exposure in the manner already described for prints on paper. These tests should be made on strips cut from a plate, or, in the absence of a diamond or other glass-cutter, on the smallest size of plate which can be obtained. These should have the same emulsion number, if possible. This testing of the time of ex posure is specially necessary when printing either from very thin or very vigorous negatives. It is an absolute rule in the cinematograph industry.
The examination of the finished transparency should always be made under the normal condi tions of its ultimate presentation. Only by very long experience is it possible to judge, from the transparency held in the hand before a well lighted white diffuser, the effect which it will produce when projected on the screen.
569. Development of Transparencies. The determination of the duration of development by the Watkins method (§ is not applicable to transparencies for lantern projection. It is, in fact, the light tones of the image which are the most important ; the time of the appearance of the shadows cannot be relied upon exclusively as a guide. The use of this method for the development of prints on bromide paper 565) is admissible only because the image of the light tones possesses the strength desired at the same time as the shadows attain their maximum density, provided that a paper is used of which the gradation is suited to the contrast of the negative. This does not apply in the case of lantern transparencies when the same emul sion is used with negatives of very different character.
Development must be judged by inspection ;' it is not necessary to examine the plate by transmitted light until the light details are visible by reflection.
Most methods of obtaining coloured images by toning also act as intensifiers. It is therefore necessary to curtail the development of trans parencies which are to be toned ; in order to obtain full details in the light tones under these conditions, a somewhat greater degree of ex posure should be given.
All developers suitable for negatives or bromide papers can be used for lantern trans parencies after adjusting the proportion of bromide. This proportion varies, however, and in certain cases can lie utilized as an aid in securing the contrast desired.
The equipment and methods of working are the same as those already described for negatives on plates or films.2 570. Making of Duplicates. In making a, positive transparency for obtaining a dupli cate negative, by printing from the positive (.§ 444), all the operations in making the trans parency and the second negative must be reg ulated according to the result desired. The new negative may be required to be as true a reproduction of the original as possible, or a reproduction possessing greater or less contrast. As a general rule, only an inferior result can he obtained from a positive which is of clear and bright appearance. It is necessary to realize
that the extreme parts of a scale of tones are invariably more or less sacrificed. Consequently, it is desirable to avoid, as far as possible, utilizing the tones corresponding with the regions of under-exposure and over-exposure in the charac teristic curve (§ 202). Thus, the intermediate transparency and the final negative should he so fully exposed that the most transparent parts of the image are represented by a distinct grey tone of a density about 0-5.
In order to avoid local irregularities conse quent on stopping development before the gamma is near its maximum, it is desirable to select emulsions of which the maximum gamma is comparatively low. Emulsions of low gamma. are generally appreciably granular. For cine matograph transparencies, films are used of which the emulsion of the transparency type is stained with a yellow dye which limits the pene tration of the light into the emulsion, and, consequently, the maximum contrast ( J. G. Capstaff, 1027). With such films the contrast may be still more reduced when printing the intermediate positive or the new negative by using a light filter by a more or less intense violet filter.
For obtaining a transparency with reduced contrast, development should be carried out in a diluted solution with a small proportion of bromide. Development can then be stopped as soon as the details in the lights are sufficiently marked on the grey tone which represents pure white. To secure a transparency with increased contrast, development may, on the contrary, be prolonged until it is no longer possible to see the image by transmitted light in the dark room, even in those parts representing the high-lights. It is, however, very seldom that the maximum contrast possible with a trans parency plate needs to be utilized. In such a case, the transparency can still be intensified, after having taken a trial print---negative—on a rapid bromide paper.
From the intermediate positive, the final negative can be obtained in a similar manner and by taking the same precautions.
Experience has demonstrated that by making systematic preliminary trials, or automatically by previous sensitometric tests (H. Schneeberger and L. Lobel, 1925), duplicate negatives can be obtained which conform to all required condi tions, and especially in giving copies which are identical in every respect with those printed from the original negative.
By superimposing the intermediate trans parency on the original negative, each tone will be complementary, and if the true contrast has been secured, the combination will show an almost even density. If the negative pre dominates, the gamma of the positive is less than unity, and, in order to obtain a new nega tive exactly corresponding with the original, development must be taken further than the development of the transparency. The inverse procedure should be adopted if, in superimposing the positive and the original negative, the positive predominates.