426 Dr. W. J. Russell. On the Action exerted by certain 



through different solid or liquid media. Glass, even of the thinnest 

 kind, was found to stop the action, but many other substances allow 

 of its transmission. For instance, the action takes place readily 

 through celluloid, sheet gelatine gutta-percha tissue, collodion, vege- 

 table parchment, real parchment, gold-beater's skin, tracing paper, 

 and no doubt many other bodies. With all these bodies experi- 

 ments have been made by placing the medium first in contact with 

 the zinc and the photographic plate, then by introducing a screen so 

 as to prevent the medium from touching the zinc, and then placing a 

 second screen so that neither zinc nor plate were in contact with 

 the medium. The screens were made of different materials, most 

 commonly of either white cardboard or sheet indiarubber, and of 

 different thicknesses. The details of each experiment need not be 

 here described, but the general results obtained are that with thin 

 sheet gelatine, either red, green, or blue when laid on the zinc, the 

 action readily passes through, and a good clear picture of the surface 

 of the zinc is obtained, and even with two sheets of gelatine a 

 similar effect is produced. With thick sheet gelatine interposed 

 the action on the plate still takes place, but of course the ex- 

 posure must be longer. Warm solutions of gelatine were painted 

 on polished zinc and allowed to harden; the action took place 

 through such layers as readily as through the films. With screens 

 used as before described to prevent contact the gelatine still allowed 

 the action to take place through it. Thin sheets of celluloid about 

 0*28 mm. in thickness allowed the action to take place through them,, 

 and sheets 0'8l mm. in thickness also allowed the action to be trans- 

 mitted. Again, gutta-percha tissue was found to act in the same 

 kind of way as the gelatine and celluloid. The other media experi- 

 mented with, although possibly not so uniform and continuous in 

 structure as the foregoing, also allow this action to be transmitted to 

 them. 



Gold-beater's skin and tracing paper both allow the action readily 

 to pass through, and pictures of the zinc are readily obtained. If 

 either of these bodies be placed between a piece of perforated zinc 

 and the plate, the perforations are very distinctly shown, or if they 

 be placed between a double screen with corresponding holes cut, a. 

 picture of the holes is readily obtained. 



Both vegetable and real parchment are also transparent to this, 

 action, but not so much so as the previously-mentioned substances : 

 the vegetable parchment is more transparent than real parchment. 

 When in contact w r ith the zinc a picture of the zinc surface ia 

 obtained, but this is somewhat modified by the substance of the 

 parchment. 



If different kinds of ordinary papers, such as writing and drawing- 

 papers, be interposed between polished zinc and a photographic 



