I have shown that the color-sensitiveness can be produced by treatment with an organic compound which has none of the optical properties characteristic of dyes; and that chlorophyl, which absorbs only red light, greatly increases the sensitiveness also to yellow and green.  There is, therefore, good reason to doubt if the color-sensitiveness is ever due to the optical properties of the dye or combination.

Attempts have been made to produce isochromatic gelatine dry plates which, while many times more sensitive to white light than my chlorophyl plates, shall also show the same relative color-sensitiveness.  Such plates would be very valuable but for one fact:  it would be necessary to prepare and develop them in almost total darkness.  Gelatine bromide dry plates extremely sensitive to yellow, but comparatively insensitive to red, might be used to advantage in portrait and instantaneous photography, because they could be safely prepared and developed in red light; but when truly isochromatic photographs are required, the time of exposure must be regulated to suit the degree of sensitiveness to red, which cannot safely be made greater than I have realized with my chlorophyl process.

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DISTORTION FROM EXPANSION OF THE PAPER IN PHOTOGRAPHY.

The effect of the unequal expansion of paper, when wetted, in causing distortion of the photographic image impressed upon it, has, in the case of ordinary photographs upon albumenized paper, been well recognized; but the extent to which such distortion may exist under different treatment is worthy of some special consideration, particularly with reference to the method of printing upon gelatinized paper, which has been thought by some likely to supersede the method now usually employed with albumenized paper.

When a print upon the ordinary photographic (albumen) paper is wetted, the fiber expands more in one direction than in the other, so that the print becomes unequally enlarged, very slightly in one and much more so in the other way of the paper.  When the paper is dried without any strain being put upon it, the fibers regain very nearly their original dimensions and position, so that the distortion which has existed in the wet condition nearly disappears.

If the photograph is cemented, while in the expanded condition, upon a rigid surface, the distortion then existing is fixed, and rendered permanent.  Such a cementation or method of mounting is that which has been generally adopted, and the consequence has been that every now and then complaints have justly been made of the untruthfulness—­owing to this particular distortion—­of photographs; productions whose chief merit has often been asserted to consist in their absolute truthfulness.  This distortion is very manifest when, in a set of portraits, some of the prints happen to have been made in one direction of the paper, and others with the long grain the other way.  I have known a case where a proof happened to increase the face in width, and all the other prints increased it in length.  Of course, neither was correct, but the proof had been accepted and liked, and the remainder of the set had to be reprinted with the grain of the paper running in the same direction as that in the first one which had been supplied.