138 Proceedings of Royal Society of Edinburgh. [sess. 
essential that a thick coating should he spread upon the material 
forming the screen. This probably indicates that the phenomenon 
is not a surface one, but that the rays pass deeply into the 
salt. In my experience, however, it is better not to crush the 
crystals too much, although a certain amount of fine division 
must take place before it can be properly and evenly attached 
to the surface. 
The salts may be placed on paper, probably black being 
better than ordinary paper ; or again, on muslin screens, or thin 
wood or vulcanite. For practical purposes I find paper more 
convenient. 
To obtain the best results, the Crooke’s tube should be enclosed 
in black paper or cloth, so as to do away with the effect of 
the ordinary light in the tube. Moreover, the room should be 
thoroughly dark, and the spark of the interrupter of the coil 
should also be covered. 
With regard to the use of the screen in a dark room, it may 
be here pointed out that some confusion exists, in the popular 
mind at least, owing to the description of Salvioni’s cryptoscope. 
It need hardly be pointed out here that Rontgen himself not 
only photographed the shadows, but proved that images of deep- 
seated structures could be obtained by means of fluorescent 
screens. All that Salvioni has done is to arrange a small box 
so as to do away with the dark chamber, but it is not as 
efficient. Salvioni himself admits, in his original paper, that he 
has done nothing which could not be deduced from the original 
experiments. 
The screens may be prepared in different ways, but I have 
found the following the best : — A quantity of barium-platino- 
cyanide is rubbed in a mortar until it forms a fine powder, 
and is then mixed with a solution of mucilage until a thickish 
yellow fluid is obtained of a consistency capable of being poured 
upon the paper, much as one would pour on collodion after the 
old wet plate photographic process. This when allowed to dry 
slowly gives satisfactory results. Another way is to stretch a 
piece of paper on a wooden frame, next to paint it over with 
a solution of gum, and through a metal sieve to dust on the 
.powdered crystals of the salt to be employed. The coating should 
