i:)2 



ims(:()Vi:hy 



obtain purple ; but the results are not so good in this 

 case, as the Prussian blue is not a peacock blue and 

 absorbs some green. 



It is the three complementarics. yellow, magenta, 

 and peacock blue, that are used in three-colour printing. 

 as may be seen by examining any three-colour print 

 with a lens. First of all a yelloVv impression is made 

 on paper ; then on the top of this a magenta or carmine 

 impression, and finally on the top of this a peacock blue 

 impression. Magenta on yellow gives red. peacock 

 blue on yellow gives green, peacock blue on magenta 

 true blue, while all three inks on the top of one another 

 give black. 



The Three Negatives. — There are many different 

 processes of colour photography and variations of the 

 different processes, some only of historical interest and 

 others only of theoretical interest. The processes are 

 still in course of development, and in the British 

 Journal Photographic Almanac those interested will 

 find references to investigations being carried on at 

 present. But the practical methods hitherto developed 

 which aim at a complete rendering of the colours all 

 require three different photographic negatives. One 

 highly successful method uses only two negatives, 

 namely the Kinemacolor method of cinematography, 

 but with two negatives the colours are not rendered 

 so accurately as with three, and consequent!}^ three 

 negatives are generally taken. 



In taking an ordinary photograph, the picture is 

 focused on the plate in the camera, and the plate is 

 then exposed, developed, and fixed. The picture 

 produced in this way is called the negative, because 

 when it is held up to the light, it gives the rendering of 

 light and shade reversed. The parts of the picture 

 which should come out light are dark : the sky, for 

 example, comes out black, so do white collars, while 

 boots come out white. Now, in the case of an ordinary 

 photograph, both visible light and invisible rays fall 

 upon the photographic plate, red, green, and blue 

 visible light, and ultra-violet invisible raj^s. But the 

 ordinary photographic plate is sensitive only to the 

 ultra-violet, the blue, and partly to the green. It is not 

 sensitive to the red. Red letters on a dark background 

 would not come out at all in the negative. It is because 

 the plate is not sensitive to red that it can be developed 

 in a red light. The single negative obtained in ordinary 

 photography is produced by ultra-violet, blue, and some 

 green rays all superimposing on the plate and producing 

 a joint picture. Colour photography replaces this single 

 negative by three separate negatives, one produced by 

 each of the three primary colours, red, green, and blue. 

 The plates employed must be sensitive to red and 

 green as well as to blue. So-called orthochromatic 

 plates won't do ; they, like Kodak and Ensign films, 

 are fully sensitive to the green, but not to the red. As a 



consequence of employing plates sensitive to red light, 

 no red lamp can be used in the dark-room. The plates 

 must be developed in darkness. 



The three separate negatives may be taken (i) with 

 an ordinary camera one after the other. In this case 

 neither camera nor object must move during the 

 three exposures. Or (ii). the three separate negatives 

 may be taken simultaneously by means of a special 

 camera provided either with three lenses or with an 

 arrangement of mirrors. Or (iii), by means of a screen 

 plate the three negatives may be taken simultaneously 

 on the same plate with an ordinary camera ; this is the 

 method employed in the Lumiere and Paget processes. 



Method used in Three-colour Printing. — The first 

 method is probably the easiest to understand, so we 

 shall describe it first. A quarter-plate camera, for 

 example, is set up before a vase of flowers, and a red 

 filter, a sheet of coloured gelatine contained between 

 two glass plates that transmits only the red rays, is 

 placed in front of the lens. Then an exposure is made. 

 Next the plate is changed, a green filter placed in front 

 of the lens, and another exposure made. Finally the 

 plate is changed again, a blue filter placed in front of the 

 lens, and another exposure made. We thus obtain 

 three negatives, one made by each of the three primary 

 colours. Now let a glass positive be made from each 

 of these negatives, and let these positives be used as 

 lantern slides and projected on a screen by three 

 separate lanterns, each in the colour in which the 

 corresponding negative was originally taken. A 

 positive is a negative made from a negative, and con- 

 sequently has the correct rendering of light and shade. 

 The arrangement of lanterns is similar to the one 

 described in connection with the mixing of colours, 

 only instead of projecting three discs of light we are 

 projecting a red, a green, and a blue picture. If now 

 the red, green, and blue pictures are superimposed, we 

 shall have a picture of the vase of flowers on the screen 

 in approximately its natural colours. This is referred 

 to as the additive process of reproducing the colours. 



Again, if we prepare half-tone blocks from each of 

 the three negatives, and make impressions of these 

 blocks on a sheet of paper, each in the colour comple- 

 mentary to the filter through which the corresponding 

 negative was taken, e.g. the impression from the block 

 made from the negative taken through the red filter is 

 in peacock blue, then we shall have a print on the paper 

 of the vase of flowers in its natural colours. This is 

 the method employed in commercial three-colour 

 work. It is referred to as the subtractive method of 

 producing the colours. 



It is unfortunately not possible in Discovery to 

 give an illustration in colours, but the adjoining ten 

 diagrams which the reader can colour for himself may 

 help to make the matter clearer, (i) is the object, a red, 



