COLOR PHOTOGRAPHY. 315 



To go into anything like detail on this complex and still debated 

 part of the science of color photography would be quite impossible— 

 and I am glad for your sakes that it is so, for only a strong expert 

 like Mr. Ives or Mr. Sanger Shepherd could lead you aright there. 

 Suffice it to say that at this point the judgment of the human eye is 

 the final court of appeal, and no conclusions based upon anything less 

 than large practical experience can be deemed final. We arc still in 

 the purely empirical stage of knowledge as to the physical connection 

 between the color and the actinic power of any given light. We are 

 not even sure that a given color of a given intensity has a constant 

 actinic power on a given film. What we see here is the result of Mr. 

 Ives's immense practical study. 



His Ivromskop introduces the three colors to the eye, not by super- 

 position, which, as you will readily see, would put three extinguishers 

 on the top of each other, but still by true corn-position. The mirrors 

 (sheets of transparent glass) which do this are models of inventive 

 power. They are not clear glass, but tinted, and the reason for this is 

 a matter of refined delicacy. The already green image passes through 

 a transparent green glass, placed on a slope which thus serves on its 

 front face as a mirror for the blue image. The latter would be 

 reflected from the back surface also if the glass were clear white; but 

 being green, it absorbs the second reflection sufficiently (in the double 

 passage of the blue light) to make it innocuous. The composite blue- 

 green image passes on to the eye through another sloping transparent 

 mirror placed to reflect the red image in the same line of sight. The 

 same danger of a double red image is avoided here by tinting the 

 mirror blue-green, which lets the blue-green image pass, but kills the 

 red light which endeavors to get twice through it. The perfection of 

 the register is thus preserved. 



Of a cognate character, but very different in its method, is Mr. 

 Sanger Shepherd's process, in which three 1 differently colored films 

 are superposed one on the other in a single transparency. They are 

 all positives without any opaque silver deposit to block out light. 

 The only gradation is from clear white to the deepest color of the dye 

 on each film — superposed they act as absorbents, and so effect the same 

 fallibly, as far as I am aware; but, assuming it to be true, see what it 

 means; nothing less than this, that we have, by the infinite delicacy of 

 photography, obtained a definite ocular demonstration of the precise 

 seat of power which ether waves have over chemical compounds. If 

 I hesitate in committing my own belief to this explanation (and my 

 belief is a matter of absolutely no importance to anyone else) it is 

 because it is not inconceivable that the disruptive action which does take 

 place may, after all, occur close round the spot where the stationary 

 ether of the node, and therefore the matter which is affected, is under 

 alternate tension and relaxation. However that may be in physics, 



