DISCOVERY 



153 



green, and blue flag nailed to a black stick, placed 

 against a white background. (2), (3), and (4) are the 

 negatives made through the red, green, and blue filters, 

 i.e. by each of the three primary' colours. (5), (6), and 

 (7) are the positives made from these negatives. (8), 

 (9), and (10) are the impressions made by the three 

 blocks prepared from the three negatives. 



When the three negatives are taken simultaneously, 

 there is no difference in principle or procedure from the 

 case in which they are taken one after another. 



Lumierc and Paget Processes. — The Lumiere screen 

 plate is an ordinary colour-sensitive plate covered with 

 a layer of flattened starch grains. These grains are 

 transparent, are coloured red, green, and blue, and are 

 well mi.xed over the whole surface of the plate. The 

 grains act as filters. They are invisible to the eye, but 

 can be seen on e.xamining the plate with a microscope. 

 It does not matter where the filter is, as long as it comes 

 between object and image. In the arrangement 

 previously described it was in front of the lens ; here 

 it is on the plate. The light cannot get to the silver 

 salt without passing through a filter. All the parts of 

 the plate behind the red grains form a negative similar 

 to that taken through the red filter in the method 

 previously described. Only here the negative, instead 

 of being continuous, forms a mosaic ; it is mi.xed up 

 with pieces of the other two negatives, and the area of 

 each is less than one-third of the area of the plate. 



After exposure the Lumiere plate is developed and 

 reversed. The image in the emulsion is converted into 

 a positive by treating it with special solutions. The 

 same plate that was the negative becomes the positive. 

 We obtain, therefore, one plate which is a combination 

 of the three positives produced by the method pre- 

 viously described. They are in the form of a mosaic, 

 all mixed up. If this positive is put in a lantern and 

 projected, it gives a picture of the original object in its 

 natural colours. If it is held up to the window, it acts 

 as a transparency giving a picture of the object in its 

 natural colours. But there is no easy method by which 

 it may be converted into a coloured print on paper. 



The Paget method differs from the Lumiere method in 

 having the screen on a separate glass plate from the 

 sensitive plate ; consequently one screen may be used 

 with a number of plates in succession. The screen is 

 covered with little red, green, and blue rectangles 

 forming a geometrical pattern. It appears colourless 

 to the eye, but the rectangles can easily be seen under 

 the microscope. They are much larger than the 

 starch grains in the Lumiere screen. The screen is 

 held in the shde with its surface in contact with the 

 plate, .\fter the plate is exposed, it is developed, ILxed, 

 and a glass positive made in the usual manner. This 

 glass positive is then combined with a screen, similar 

 to that through which the original exposure was made. 



The screen and positive must be carefully adjusted, so 

 that they are accurately in register. Each part of the 

 positive must be opposite a rectangle of the same 

 colour, as the exposure of the corresponding part of the 

 negative was made through. Wlien positive and screen 

 are combined in this manner, they form a coloured 

 transparency which can be viewed by holding it up to 

 the window, or which may be projected as a lantern 

 slide. There is no easy way of converting this trans- 

 parency into a coloured print on paper. 



Anyone possessing a camera that takes one of the 



Red Cr«n BIu 



mimai 



!'"=■ ■■ Fic. 3- I-IC. 4. 



Negative through red Negative through qrccri Ne.;ativc through blue 



inter. ;it.:r. ,-,iti.r. 



Pin 



Positive tor projection Positive for projection Positive for project: 



White 



FIG. 8. Fig. 9. Fig. 



The three impressions in three-colour printing. 



Standard sizes of plates can make coloured trans- 

 parencies with a little patience by either the Lumiere 

 or Paget processes, and many amateurs dabbled in 

 these processes in the days before the war. But 

 the drawback is the impossibility of making coloured 

 prints. And there is also a difficulty in obtaining 

 suitable objects to photograph. An English city 

 contains comparatively little colour ; perhaps the only 

 bright colour in a street is a red letter-box or the 

 coloured side-boards of a tram-car. And in the countrj', 

 except at certain seasons, the colours are never 

 saturated. So possibly the best rendering may be given 

 by a sepia monochrome sHghtly tinted by hand. 



The method used for three-colour printing is, of 

 course, the method by which three separate negatives 



