808 Captain Ahney [Feb. 25, 



remembered tbat the negatives obtained have to be converted into 

 positives ; and further, that for effective working all three negatives 

 must, in ordinary circumstances, be obtained on one plate, and by 

 one length of exposure. Mr. Ives has worked this out with a wonder- 

 ful degree of exactitude, and his camera can be examined in the 

 Library after the lecture to show the manner in which he has 

 accomplished it. He has aimed at getting a perfectly graduated 

 negative with each colour screen, and in the positives from them there 

 are absolutely transparent parts, thus securing the maximum bril- 

 liancy. 



These positives are backed by colour screens chosen to imitate 

 the three colours used by Clerk Maxwell in his colour mixture equa- 

 tions. 



Now, having explained the principles of the three-colour photo- 

 graphy, I will get Mr. Ives to throw three or four of his pictures on 

 the screen, and I have to thunk him for his ready acquiescence in 

 responding to my request for his help to-night. It is a pleasure 

 to acknowledge that Mr. Ives has been the pioneer in this colour 

 photography, working on exact principles, which he has applied to 

 practical purposes. 



In connection with the same subject we have the more recent 

 process due to Professor Joly, of Dublin. Instead of taking three 

 negatives and from them three transparencies, he combines the three 

 in one. To take his negatives he observes the same general principle 

 as that already enunciated, for he places in contact with his sensitive 

 plate a screen consisting of a series of orange, green and blue lines 

 ruled on w^hite glass and touching one another ; each line is ^i^ of an 

 inch in width. Every third line is a colour screen in orange, the 

 next line and third from it a green, and the remaining ones blue. 

 To tone down the excess of blue in daylight, the lens is covered with 

 a pale yellow screen. The one negative is therefore a mixture of 

 three colour negatives. A transparency is taken in the usual w^ay, 

 and by placing in contact with it a screen ruled in red, green and 

 blue, the red lines occupying the position of the orange line in the 

 taking screen, the green the green, and the blue the blue, we have a 

 representation in colour of the original object. [The taking screen, 

 the viewing screen, and a negative and a positive were shown, as also 

 a selection of finished pictures taken by Professor Joly.] 



Suppose we take one set of Ives' negatives and make duplicate 

 prints from them in bichromated gelatine, we should get, on develop- 

 ment, transparent gelatine of different thicknesses. where the light 

 had most acted the film would be thickest, and where no light had 

 acted the gelatine would be practically absent, and the intermediate 

 intensities of light acting would give intermediate thicknesses of 

 gelatine. We may dye one set of gelatine prints with a transparent 

 red, a transparent green and a transparent blue, to imitate the viewing 

 screens, and if these were superposed we should find a very different 

 result to that obtained by triple projection. What ought to be black 



