ISOTYPIE A process of half-tone negative making pro posed by Vittorio Turati, and expounded in an elaborate treatise. The system depends on the use of variously shaped diaphragms in con junction with a suitable screen distance. The latter is determined by means of what the inventor termed a " finder " stop, having two small openings placed at a mathematically deter mined distance apart. The image of this stop is viewed by means of a microscope through the ruled screen, whilst the latter is moved to and fro until 4 position is found where the images of the two openings coincide in the focal plane. The variously shaped diaphragms can be put in the lens, and different patterns of dot forma tion are obtained on the negative, although the ordinary cross-ruled screen has been used throughout. The process has not come into commercial use.
" IT " Sir William Abney in his presidential address before the Royal Photographic Society, in October, 1896, proposed the term " It " (the initials of intensity and time) as an expression of the unit of exposure. He used the term pro visionally only, and suggested that a unit might be called " talbot," just as in other branches of physics the units were named after pioneers, as, for example, the " watt," the " volt," and the " ampere." P. E. Ives, of Philadelphia, U.S.A., may be considered as one of the pioneer workers in practical three-colour work, as it was mainly by his efforts that the true theory of the processes was recognised and of recent years reduced to practical results. He laid down the principle, though it had already been enunciated by Clerk Maxwell and Ducos du Hauron, that the object to be reproduced in colours should be photo graphed through three separate screens or colour filters, which should give negatives that repre sented by light and shade the degree to which light coming from different portions of the sub ject excites a single primary colour sensation in the eye ; and that for projection these three photographs should be projected simultaneously upon a screen, each by light which excites only the primary colour sensation which it represents, and in such manner that the three-coloured images are superimposed. This involves the
production of one photograph by the joint action of the red, orange, yellow, and yellow-green rays, but chiefly by the orange, so as to represent the effect upon the red sensation ; another by the joint action of the orange, yellow, yellow green, green-blue rays, but chiefly by the greenish yellow rays so as to represent the action of the green sensation ; and a third photograph by the joint action of the blue-green, blue, and violet rays, but chiefly by the blue rays to represent the action of the blue sensa tion. Positives from negatives were projected, the first by pure red light, the second by pure green light, and the third by blue-violet light. Several cameras of varying types have been devised by Ives to obtain the negatives, some in which the image formed by one lens was split up by means of mirrors into three images, and others with three lenses. In his latest type only one lens is used, and a reflector splits the image up into two parts, and one image is received by one plate, and the other two by a plate and a celluloid film placed surface to surface with a colour screen in between.