SCIENCE IN THE INDUSTRIAL WORLD 



plate. The powder originally dusted on the plate, 

 which has lost its color, is washed off." 



Results in color photography have also been obtained 

 by the use of prisms or diffraction gratings instead of 

 dyes or pigments. Professor Lippmann, of Paris, 

 whose direct "interference" process we noted above, 

 has devised a method employing the minute spectra 

 of prisms. The apparatus employed is similar to the 

 photographic spectroscope, except that "the single 

 slit of the spectroscope is replaced by a series of slits 

 very close together consisting of fine transparent lines 

 ruled five to the millimeter. This grating is fixed at 

 one end of a solidly built box, the other end carrying the 

 photographic plate, and between these is a converging 

 lens, in front of which is a prism of very small angle. 

 The object to be reproduced is projected on the grat- 

 ing, illuminated with white light. The light passing 

 through the prism and lens falls on the sensitive plate, 

 producing a negative in black and white which under 

 the lens appears lined, each line divided into small 

 zones, which are parts of an elementary spectrum. 

 If the negative be now replaced in its original position 

 and illuminated by white light, the image of the object 

 photographed is seen in colors which are complemen- 

 tary to those of the object; the latter appears in its 

 own proper colors when the negative is replaced by 

 a positive." 



It will be seen that the apparatus in which the ex- 

 posure was made must also be used to get a color image 

 of the photographed subject. M. Andre" Che'ron, of 

 Paris, has more recently improved the process, to some 



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