proceedings of the polytechnic association. 343 



Printing in Colors. 



Mr. Fisher.— It is well known that the success in printing in colors has 

 been very imperfect, and I suppose that this is owing to the want of artistic 

 knowledge and taste on the part of the operators. Brewster ascertained 

 that all colors are formed by the combination of three: red, yellow and 

 blue, which he called the primary colors. Now, I suppose that if these 

 three were properly blended, all colors and all shades of color might be 

 produced. Perhaps black might also be employed with advantage. I 

 suppose that nine-tenths of the colors and shades employed in portrait 

 painting might be produced from red, blue and black. 



Dr. Stevens. — Probably the man who first printed in colors was Faust. 

 Before the art of printing with movable types was invented it was customary 

 to ornament the manuscript books with illuminated letters of various 

 colors, and in order to make the printed books as nearly like those in use 

 as possible, the printers undertook to print their initial letters in colors, 

 and they produced some very fine results. But the practice after a while 

 fell into disuse. At the present time, printing in colors is carried on in this 

 country with very satisfactory results, though not so well as in England 

 and Germany. Maps especially are printed in this way, so as to rival 

 those colored by hand. Prof. Rogers, formerly of this country, but at 

 present of Edinburgh, had a geological map printed in this way, which ia 

 nearly, if not quite, equal to any colored by hand. Each color is put on 

 by a separate stone; though after all of the sheets are printed in one color 

 from a stone, the color may be wiped off, and the same stone may then be 

 prepared for another color, with which all of the sheets may be printed. 



Mr. Gavitt. — Mr. President: I received your invitation so short a timo 

 before the meeting that I have made no preparation, and my remarks musl; 

 be very desultory. The American Bank Note company print their bills ir« 

 colors as a guard against counterfeiting by the photographic process. H 

 a bill is printed partly in red, the counterfeiting photographer must remove 

 the red before he photographs the rest of the bill, and the red must then bo 

 printed in. The colors formerly used could all be readily removed, but it 

 was suggested by one of the most eminent chemists in the world, Mr, 

 Sterry Hunt, that the sesquioxyd of chromium would be found as permanent 

 as the black carbon ink. We have accordingly adopted this pigment, and 

 hence the great quantity of green you see in modern bank notes, especially 

 in the United States treasury notes. The sesquioxyd of chromium resists 

 the action of all acids except boiling nitric acid, and that destroys the 

 texture of the bill. The only way in which it can be removed is by saponi- 

 fying the oil which is employed as a medium to attach it to the paper, and' 

 as the same medium is used for the carbon ink, if one is removed they both 

 go together. We print the green over the black, and this we consider a 

 perfect safeguard against counterfeiting by photography. 



Prof. Seely. — I think, Mr. Chairman, that this apprehension of counter- 

 feiting by photography is a bugbear. I have frequently heard of bank 

 notes that were counterfeited by photography, and I have been told that if 

 I would go to this place or that place, I should see one so perfect that it 

 could not be distinguished from the genuine note. The narrators seem not 



