REPRODUCTION OF ILLUSTRATIONS 



in such an instrument as the spectroscope, as is well 

 known, this analysis of the composition of substances 

 may be made. Acting upon the knowledge of these 

 facts, attempts were made to produce transparent "fil- 

 ters ' ' which, when used in connection with a photographic 

 plate, allowed one of the primary colors of a picture to 

 act upon the plate while excluding the other two. These 

 experiments finally proved successful with the result 

 that practical filters were made, allowing the transmission 

 of the rays of any one of the three primary colors while 

 excluding the others. Such filters were made in various 

 ways, sometimes of transparent colored glass, or glass 

 coated with some substance of the required tint, or again 

 of a hollowed glass containing a liquid of the proper 

 color. 



In using these color filters, or screens, in the actual 

 process of three-color work, the photographer makes 

 three separate negatives, taking each from exactly the 

 same position, one negative being made with a yellow 

 filter placed between it and the picture to be reproduced, 

 a second with a blue, and the third with a red. These 

 negatives are developed and three separate half-tone 

 blocks made from them, each block representing by its 

 gradation of light and shadow the amount of yellow, 

 blue, and red, respectively, contained in the picture. 

 The blocks are then placed in the press and printed 

 successively upon the surface of paper, using yellow, 

 blue, and red ink, the result being an exact reproduction 

 of the original picture with the colors faithfully produced, 

 if sufficient time and skill are used. 



Theoretically it makes no difference which color is 

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