172 
Ordinary Meeting-, November 29th, 1859. 
Dr. R. A. Smith, F.R.S., Vice-President, in the Chair. 
A letter from Mr. Dyer was read, relative to Mr. Jevons’s 
Paper. Mr. Dyer is led to conclude from the facts which 
have been ascertained respecting the distribution of gold, 
that that metal forms a great part of the material of the 
earth’s crust now in an incandescent state. 
Mr. F. O. Ward laid before the Society an instrument 
termed a Pseudo-diascope, and a Paper setting forth its 
construction and use, and the principle it is designed to illus- 
trate. By means of this instrument an aperture transmitting 
light is made to produce on one eye an isolated impression, 
while the other eye is directed to an opaque body, such as the 
hand held before it. The image of the aperture is then found 
to be transposed, and its perception ceases to be assigned to 
the eye by which it is really seen ; the elfect being that a 
perforation appears in the opaque body, through which the 
light seems to shine upon the eye by which this is viewed. 
The principle illustrated by this instrument, according to the 
Author’s view, is the essentially goniometrical and deductive 
nature of the visual act, whenever the distances of bodies are 
perceived, and their relative positions in space assigned. A 
Pseudo-diascope was presented to the Society by the Author, 
and the singular illusion produced by it was verified by the 
members present. 
Mr. F. O. Ward subsequently laid before the Society a 
plan of his for diminishing the liability of powder mills to 
explosion, and referred to a correspondence between himself 
and Dr. Faraday on the subject. The plan in question 
consists in supplying to those portions of powder mills in 
which the powder is treated dry, an atmosphere incapable of 
supporting combustion — preferably carbonic acid gas — so as 
Proceedings— Lit. & Phil. Society — No, 5.— Session, 1859-60. 
