June 23, 1899 ] 



SCIENCE. 



861 



rection, or, in other words, to the same spot 

 on the screen behind the lens. 



Suppose, now, we have a glass plate with 

 a design of a tulip, with its blossom ruled 

 with 2,000 lines to the inch, its leaves ruled 

 with 2,400 and the pot in which it is grow- 

 ing ruled with 2,750 lines, and place this 

 plate before the lens. On looking through 

 the hole we shall see a red tulip with green 

 leaves growing in a blue pot. Thus we see 

 how it is possible to produce a colored pic- 

 ture by means of diifraction lines, which are 

 in themselves colorless. Those portions of 

 the plate where there are no lines send no 

 light to the eye and appear black. 



We have, now, to consider how this prin- 

 ciple can be applied to photography. That 

 photographs which show color on this prin- 

 ciple can be made depends on the fact that 

 a diffraction grating can be copied by con- 

 tact printing in sunlight on glass coated 

 with a thin film of bichromated gelatine. 

 The general method which I have found 

 best is as follows. Three gratings ruled on 

 glass with the requisite spacing were first 

 prepared.* To produce a picture in color, 

 three negatives were taken through red, 

 green and blue color filters in the usual 

 manner. From these three ordinary lan- 

 tern-slide positives were made. A sheet of 

 thin plate glass was coated with chrom 

 gelatine, dried, and cut up into pieces of 



*These gratings were ruled for us on the dividing 

 engine at Cornell University, through the courtesy of 

 Professor E. L. Nichols. ' 



suitable size ; one of these was placed with 

 the sensitive film in contact with the ruled 

 surface of the 2,000-line grating, and the 

 whole covered with the positive represent- 

 ing the action of the red light in the picture. 

 An exposure of thirty seconds to sunlight 

 impressed the lines of the grating on the 

 film in those places which lay under the 

 transparent parts of the positive. The 

 second grating and the positive represent- 

 ing the green were now substituted for the 

 others and a second exposure was made. 

 The yellows in the picture being transpar- 

 ent in both positives, both sets of lines were 

 printed superposed in these parts of the 

 picture, while the green parts received the 

 impression of 2,400 lines to the inch only. 



The same was done for the blue, and the 

 plate then washed for a few seconds in 

 warm water. On drying it appeared as a 

 colored photograph when placed in front of 

 the lens and viewed through the hole in the 

 screen. Proper registration during the 

 triple printing is secured by making refer- 

 ence marks on the plates. A picture of 

 this sort once produced can be reproduced 

 indefinitely bj' making contact prints, since 

 the arrangement of the lines will be the 

 same in all of the copies as in the original. 

 The finished picture is perfectly transpar- 

 ent and is merely a diffraction grating on 

 gelatine with variable spacing. In some 

 parts of the picture there will be a double 

 grating, and in other parts (the whites) 

 there will be a triple set of lines. Having 

 had some difBculty in getting three sets of 

 lines on a single film in such a way as to 

 produce a good white, I have adopted the 

 method of making the red and green grat- 

 ings on one plate and the blue on another, 

 and then mounting the two with the films 

 in contact. It is very little trouble to mul- 

 tiply the pictures once the original red- 

 green grating picture is made. 



The pictures are viewed with a very sim- 

 ple piece of apparatus shown in Fig. 4, 



