SCIEXCE- G OSSIP. 



■ ■ A triangle was thus iUuminated, so that the 

 pure colours appeared at its angles, while the rest 

 of the triangle contained the various mixtures of 

 the colours, as in Young's triangle of colour. 



11 The graduated intensity of the primary colours 

 in different parts of the spectrum was exhibited 

 by the coloured images, which, when superposed 

 on the screen, gave an artificial representation of 

 the spectrum. 



" Three photographs of a coloured ribbon taken 

 through the three coloured solutions respectively 

 were introduced into the lantern, giving images 

 representing the red, the green, and the blue parts 

 separately, as they would be seen by Young's three 

 sets of nerves separately. When these were super- 

 posed, a coloured image was seen which, if the 

 red and green images had been as fully photo- 

 graphed as the blue, would have been a truly- 

 coloured image of the ribbon. By finding photo- 

 graphic materials more sensitive to the less 

 refrangible rays, the representation of the colours 

 of objects might be greatly improved." 



The primary cause of the failure, then, of Clerk- 

 Maxwell's attempt was the insensitiveness of the 

 photographic plates of his day to red and green. 

 Xow, however, the case is very different, as plates 

 are commercially obtainable sensitive in some 

 degree to the whole of the visible spectrum, so that 

 all we have to do is to prepare three colour filters 

 admitting light in such proportions that we may 

 secure in our negatives densities corresponding 

 respectively to the curves shown in fig. 6. The 

 red filter must allow light to pass so as to secure 

 action upon the photographic plate from the lines 

 A to E with the maximum of action in the orange 

 between C and D. The green filter must admit 

 light so as to secure an action upon the plate from 

 the line C to midway between D and E. and the 

 blue filter negative must ad m it light so as to secure 

 action from the line E to H with a maximum of 

 action about midway between the lines F and G. 

 Until quite recently all attempts to adjust the light 

 filters to these curves were made by trial and error, 

 repeatedly photographing the spectrum through 

 various colour filters, until the action secured upon 

 the plate corresponded approximately to the curves 

 shown in fig. 6. By a modification of Sir William 

 Abneys colour sensitometer it is, however, now 

 possible to secure far greater accuracy with con- 

 siderably less expenditure of time, and it is to the 

 greater accuracy so secured that the great im- 

 provement of trichromatic work during the last 

 few months has been due. 



Light niters correctly adjusted by means of a 

 measured Abney sensitometer for use with the 

 Cadett Rapid Spectrum plate are now obtainable 

 commercially, and in fig. 9 I have endeavoured to 

 show as clearly as possible the selective action of 

 the bight filters when photographing a row of 

 squares of coloured glass. 



A (fig. 9) represents the test object, consisting of 



small squares of white, red. green, blue, and yellow 

 glasses, and also a black or opaque square. 



B represents a negative of the test object taken 

 through the red-light filter. The exposure has been 

 made so as to secure a full dense deposit upon the 

 plate by the light coming through the white 

 square ; the next square — the red — is also repre- 

 sented by a dense deposit ; but the light coming 

 through the green and blue squares has been cut 

 out by the red filter, and they are represented in 

 the negative by transparent spaces. The next 

 square — the yellow — is represented by opacity > 

 because, as we saw in fig. 7, red and green light 

 was required to make yellow : and the black square, 

 allowing no light to pass to the plate, is represented 

 by transparency. 



C. the next row of squares, represents the 

 selective action of the green-light filter : here the 

 white square is again represented by opacity, 

 the red square by transparency, the green square 

 by opacity, the blue square by transparency, the 

 yellow square by opacity, and the opaque square 

 by transparency. 



D represents the selective action of the blue- 

 light filter ; here again the white square is repre- 

 sented by opacity, the light coming through the 

 red, green, and yellow squares is cut out by the 

 filter and they are represented by transparency, 

 the blue square is represented by opacity, and the 

 black by transparency. 



Such a set of negatives thus constitutes an 

 accurate colour record of the test object, and it only 

 remains to explain how such a,record can be used 

 to reproduce the actual colours of the test object. 



This object can be achieved by Clerk-Maxwell's 

 device of making transparencies from the negatives 

 and projecting them in superposition upon a screen 

 by means of three optical lanterns, illuminating 

 the red-filter transparency by red hght, the green- 

 filter transparency by green hght, and the blue- 

 filter transparency by blue hght. But very early 

 in the history of trichromatic photography the 

 inconvenient nature of this method, with its con- 

 sequent enormous loss of hght, was felt, and in 

 1869 the able French experimenter, Du Hauron. 

 came forward with several entirely new methods 

 of synthesis. He invented the first photochromo- 

 scope, or table instrument, for optically combining 

 the three transparencies — a device which led to 

 the Kromskop of Ives. He suggested making a 

 mosaic screen by ruling alternate very fine lines 

 close together on a glass plate in transparent red, 

 green, and blue ink, afterwards re-invented and 

 commercially worked by Dr. Joly. of Dublin : but. 

 most important of all, he showed us how it was 

 possible to print from the colour record negatives 

 in transparent pigment colours and superpose the 

 prints, thereby securing as a single print a perfect 

 representation of the object, which could be seen 

 in the hand as a complete picture in colours or 

 projected upon the screen by a single lantern. 



