1898.] on the Theory of Colour Vision, &c, 807 



this gives the part occupied by the green sensation curve ; and so 

 with the blue. Evidently, then, by using the orange, blue-green and 

 blue media for the three photographs of the spectrum, we shall secure 

 three negatives representing, with some degree of exactness, the sen- 

 sation curves, though the exposures given to each one will be very 

 different. The red will require nearly one hundred times more 

 exposure than the blue, and the green an intermediate exposure. 

 On the screen we have the negatives obtained, aud also the positives 

 (Fig. 6). No. 1 was taken through the orange, No. 2 through the 

 green, and No. 3 through the blue screen. The superposed images 

 of these three positives, if backed by red, green and blue light, will 

 give us the spectrum. \Mr. Ives showed the projection on screen.] 

 The picture is fairly perfect, and exemplifies what can be done with 

 an oidiuary plate. 



What 1 wish to impress upon you is that the screens used for the 

 taking of the three different negatives must each allow a large part 

 of the spectrum to pass, whereas the colour screens used to illuminate 

 the three positives, where the images are superposed, will be more 

 efficient the smaller the part of the spectrum that is used, for if large 

 parts are used the colours will be tinged with white. This is a 

 most important point in three-colour photography. 



We have modifications of plates which allow shorter exposures 

 to be given to the green and the red. Cadett's spectrum plate (see 

 No. 1, Fig, 5), for instance, can be utilised for giving equal exposures 

 through a blue, a green and a red medium, when the white light is 

 first toned down to a pale yellow, which, however, still contains all 

 the colours of the spectrum. 



Then there are others, such as Lumiere's (see Nos. 2 and 3, 

 Fig. 5), which are sensitive to the green and yellow and red, as well 

 as to the blue, but which exhibit gaps in sensitiveness in the length 

 of the spectrum. These plates can be utilised for photographing 

 colours in nature, though they must fail for photographing the spec- 

 trum. But to atone for the gaps, the absorbing media used have to 

 be modified to effect a compromise as it were. Mr. Ives, who is the 

 inventor of the Photochromoscope, and who is present this evening 

 to show some of his wonderful results, has kindly lent me a slide 

 showing the screens with which to take the three negatives with 

 Lumiere's pan-chromatic plates. 



By modifying the screens, any plate which is sensitive to the 

 yellow and orange may be utilised, even though it is not at all, or 

 only very feebly, sensitive to tlie red. For be it remembered that the 

 colours in nature are not pure spectrum colours. A red, for instance, 

 such as this glass, contains an appreciable amount of yellow in it, 

 and the yellow will impress the plate sufficiently to answer the 

 purpose of obtaining the requisite density to represent the red. Of 

 course, if there were a red of a spectrum simplicity, it would not 

 impress the plate. Except with the ordinary plate such as I have 

 used, tbere has to be a series of compromises. Again, it must be 



3 G 2 



