1865.] Physics. 497 



colour. Experiments with these prisms have shown that the power of 

 converging the optic axes differs very considerably in individuals. 

 The pseudoscopic effects are exaggerated by using a prism to each 

 eye, but in most persons this produces a painful sensation. The ex- 

 planation of these phenomena, which Mr. Dancer offered, and which 

 seems to be the correct one, is based upon the supposition that in 

 binocular vision we estimate the distance of an object by the degree 

 of convergence of the optic axes. In these experiments, when a flat 

 surface appears concave by the interposition of the prism, the optic 

 axes are made to converge on a point situated behind the real surface, 

 and the imagination gradually removes the object to this apparent 

 distance. When the base of the prism is towards the nose, then the 

 flat surface becomes convex : in this case the optic axes cross in 

 front of the real surface, and the imagination raises the object to that 

 point. 



M. Docle, a French chemist, has introduced platinum mirrors, 

 which are greatly admired, and which present this advantage, that the 

 reflecting metal is deposited on the outer surface of the glass, and thus 

 any defect in the latter is concealed. The process, which is patented 

 in France, is described as follows : — Chloride of platinum is first made 

 by dissolving the metal in aqua regia and driving off the excess of 

 acid. The neutral chloride is then dissolved in water, and a certain 

 quantity of oil of lavender is added to the solution. The platinum 

 immediately leaves the aqueous solution and passes to the oil, which 

 holds it in suspension in a finely-divided state. To the oil so charged 

 the author adds litharge and borate of lead, and paints a thin coat of 

 this mixture over the surface of the glass, which is then carried to a 

 proper furnace. At a red heat the litharge and borate of lead are 

 fused and cause the adhesion of the platinum to the softened glass. 

 The process is very expeditious. A single baking, M. Dode says, will 

 furnish 200 metres of glass ready for commerce. It would take 

 fifteen days, he says, to coat the same extent with mercury by the 

 ordinary plan. 



Photographers have for many years been endeavouring to substi- 

 tute salts of uranium for salts of silver. One of the most promising of 

 the different suggestions for this purpose has just been made by M. 

 Liesegang. He prepares an ammoniacal citrate of uranium and mixes 

 it with a little solution of chloride of gold and a paste prepared by 

 dissolving tapioca powder in hot water. The quantity of chloride of 

 gold must be small, and the heat not too great, otherwise the gold will 

 be reduced. Spread this mixture with a sponge on paper, which takes 

 a brilliant yellow colour, similar to albuminized paper ; when quite 

 dried in the dark, place it in the positive frame. The proofs have all 

 the force and delicacy of those obtained by albuminized paper ; the 

 preparation is very sensitive, and becomes more so if slightly mois- 

 tened. The proofs come from the frame of a bluish black colour ; 

 they should not be toned, and should be fixed by being washed with 

 rain-water until the yellow colour of the paper completely disappears. 

 vol. n. 2 M 



