362 SCIENCE PROGRESS 



McDonough, in America, from 1892 to about 1898. The later 

 years of this period include attempts to make the process work- 

 able, so that the screens necessary might be manufactured in 

 quantity, but in this they were not successful. The three 

 colours were arranged regularly in lines, and were applied 

 to the glass by means of a ruling pen, though other methods 

 were tried afterwards. Dr. Joly followed Ives very nearly so 

 far as his colours were concerned, except that he sought to 

 get his colours to accord with the more recently determined 

 sensation curves. He used two colour screens — a " taking- 

 screen " that was fixed in front of the plate while the exposure 

 was being made, and a " viewing screen " which was put in 

 front of the positive for seeing the picture, and might be bound 

 up with it as a fixture if preferred. The lines were always so 

 coarse as to be obtrusive. 



Quite recently, after years of work, Mr. John H. Powrie, of 

 Chicago, assisted by Miss Florence Warner, has so far perfected 

 a triple-line method that he has been able to show many 

 excellent examples. He has overcome the difficulty of making 

 the fine coloured lines by discarding ruling methods altogether. 

 He coats the glass with a suitable colloid made sensitive with 

 potassium bichromate, and exposes this to light under a black- 

 line screen that has lines twice the width of the spaces between 

 them. The light makes the colloid insoluble in warm water, 

 so that when " developed " like a carbon print, the coating is 

 entirely dissolved away where it was protected by the black 

 lines of the screen, and remains on the glass in fine lines 

 corresponding to the screen spaces. The plate is put into a 

 solution of a green dye ; the colloid absorbs the dye, and 

 thus the green lines are formed. The colloid is made quite 

 insoluble to fix the dye. The plate is then coated again, and 

 treated exactly as before ; but care is taken that the black lines 

 of the screen hide the green lines already produced, and a red 

 dye is used. It remains to get the blue lines that shall exactly 

 fill the spaces between the two sets of lines already obtained 

 and without any overlapping of them. The plate is coated 

 again, and this time exposed to light without the black line 

 screen, but with its back to the light, so that the red and green 

 lines on the plate prevent any light effect on the coating that 

 overlies them and acts only upon the parts between. Warm 

 water dissolves away the new colloid layer over the green and 



