June 4, 1920] 



SCIENCE 



557 



Azo developing paper, preferabjy the latter 

 in each case. The coloring is secured by 

 painting directly upon the dry prints with 

 transparent dyes. The detail of the image is 

 supplied by the lines of the print itself. 

 Water-soluble aniline dyes in the colors 

 yellow, orange, red, brown, blue and royal 

 blue are commonly used. The original plate 

 is colored with the specimens before the artist 

 and, while it has been necessary to supervise 

 closely the color work on this print, a sur- 

 prising degree of skill and accuracy has been 

 developed by some of the operatives. Since 

 most truck crop disease specimens are highly 

 perishable and change considerably during the 

 time elapsing between exposure of the nega- 

 tive and the completion of prints, even though 

 held in a refrigerator, it has been found ad- 

 vantageous to register the exact colors on 

 some other print of proper color value at the 

 time of exposure, or if possible to collect 

 fresh specimens of a similar character. 



Inasmuch as the print is to serve as a back- 

 ground for the color, the kind of paper chosen 

 and the intensity of the image depend upon 

 the color effects desired. For example, the 

 printing-out paper is desirable for most yel- 

 lows, browns, and reds, while for purples, 

 blacks, and dark greens the developing paper 

 is preferable. However, the printing-out 

 paper serves very well for the majority of 

 greens and has been more extensively used. 



After the dyes are mixed and diluted to 

 secure the desired shade, the gelatin surface 

 of the print is prepared for coloring (prob- 

 ably softened and swelled) by wiping with a 

 cloth moistened with alcohol, ammonium 

 hydroxide, or more commonly saliva, and the 

 dye is applied with a brush in rather liberal 

 amounts of which the excess is removed by 

 means of a blotter. The quality of the color 

 is determined by the proportions of the dye 

 mixture and the type of paper used for the 

 print; the intensity of color is determined by 

 the dilution of the dye, the intensity of the 

 photographic image, and the length of time 

 the excess dye is allowed to remain on the 

 print before blotting. In case of error the 



color can be removed with ammoniimi hy- 

 droxide. In some instances a very small 

 amount of this substance added to the dye 

 causes the latter to spread and adhere more 

 satisfactorily. Details in white or backgrotmd 

 color, such as holes in a leaf, can be con- 

 served by coating with a paste or enamel 

 which is insoluble in ordinary solvents and 

 is removed with benzine after the coloring is 

 completed. Details in black, such as the 

 blackened veins in cabbage black rot, can 

 best be shown by the image on Azo paper. 

 After the coloring is completed, the prints 

 are run through a mordant bath to fix the 

 colors. Combinations of acetic acid, formalin, 

 and other mordant reagents constitute this 

 bath, the exact composition of which depends 

 in part upon the colors to be fixed. The 

 gelatin surface must be thoroughly wetted by 

 the solution. The prints are then rinsed in a 

 water bath, placed face downward on squeegee 

 boards, sponged, and passed through rollers 

 to remove the excess water. The prints are 

 mounted while wet on muslin or Japanese 

 paper with a cardboard flap and allowed to 

 dry on the squeegee board. 



While these colored photographs are uJti- 

 mately to be used for lithographs, it has been 

 found feasible to reproduce about ninety sets 

 of fifty duplicates each for immediate use by 

 hand coloring of duplicate prints, the original 

 colored print being used as a guide. How- 

 ever, this process is too laborious and un- 

 reliable for large scale production and the 

 colors will not endure indefinite exposure to 

 light. Colored lantern slides of a very grati- 

 fying quality have also been made. 



This process of color reproduction could 

 well be utilized in other branches of science 

 and there appears to be no reason why it could 

 not be perfected and employed by educational 

 and research institutions. The results of this 

 method of scientific illustration are far supe- 

 rior to uncolored reproductions and are, it is 

 believed, an improvement over other types 

 of color reproduction because of the accuracy 

 of detail afforded by the photographic image. 

 Such illustrations should find wide use in 



