71 



From this time onward a host of workers have been experi- 

 menting in the production of a photograph in colour. We shall 

 now consider how far they may be said to have succeeded. 



In order to attack the problem in a scientific way it was 

 necessary that certain bases should be established. From these 

 experiments were made in all directions, some in the way of the 

 interference principle, some towards diffraction and others towards 

 the pigment method. One great bar was the question of what 

 were the primary colours. Scientists were very much at variance 

 on this point, and the number of colours thought necessary to 

 reproduce all the colours of nature ranged from three to some 

 eight or nine. One or two put out the theory that red, yellow, 

 and blue were the primaries, but they at last fell into line with the 

 majority who had accepted the theory that red, green, and blue 

 were the three colours required to produce coloured photographs 

 either on glass or paper. It is usual, however, to describe the 

 red as orange red and the blue as blue violet. In practice, how- 

 ever, a fourth colour is used, viz., yellow, but this is for a specific 

 purpose which will be explained later. This brings me to a few 

 remarks on the varying wave lengths of colours in the spectrum, 

 another base from which the subject of our paper springs. The 

 activity of the waves of light at the upper end of the spectrum 

 compared with that of those at the lower end was so great that 

 -on an ordinary plate, which is practically only sensitive to the 

 blue violet rays, it was impossible to photograph a red object and a 

 Dlue one, say the sky, on one plate with the same exposure. It is 

 •clear that while this state of things existed no advance could be 

 made towards coloured photography. But the accidental discovery 

 oy Dr. Vogel opened up the whole question of dyed plates, the 

 adoption of which led to surprising results. First came a plate 

 sensitive to the green and yellow rays and known as the ortho- 

 chromatic, soon followed by a plate sensitive to the red rays and 

 described as panchromatic. What happened was a suppression of 

 the activity of the blue violet simultaneously with the acceleration 

 -of the tardy reds. The panchromatic plate thus became the photo- 

 graphic basis of the screen plate photography of to-day. The use 

 of a yellow screen is to reduce still further the activity of the blue 

 violet and ultra blue violet, which latter are not visible to the 

 human eye, but are capable of affecting the sensitive plate to 

 such a degree that a photograph has been taken by these, rays 

 alone. 



Having established these two important bases, the road to 

 colour photography becomes more clear, so we now proceed to 

 examine the ways taken in its further progress. Lippmann's 

 process of pure colour photography by interference was so com- 

 plicated that although successfully accomplished by the inventor, 

 very few people have ever seen a specimen, and no one, as far as 

 known, has succeeded in producing one. We will, therefore, only 

 consider those processes that have come into practical use. There 

 were two main lines of approach, one by the Three Colour Process 



