112 
ocular pictures for the use of lectures, &c. Some beautiful 
stereoscopic transparencies were cut in two, and each half 
placed in its position in oxhydrogen lanterns mounted with 
achromatic object glasses. The half stereographs were then 
projected in juxtaposition on a long screen, and to realize 
the proper stereoscopic effect, the members were supplied 
\fdth achromatised prismatic stereoscopes which had been 
prepared expressly for the purpose. Mr. Dancer made some 
remarks on the perfection which the lenticular stereoscope 
had attained by the employment of achromatic lenses, and 
urged the necessity of photographers and opticians agreeing 
to uniform foci for the lenses of stereoscopic cameras and 
stereoscopes, as it was only by viewing the pictures with 
lenses of equivalent foci to those with which they were taken 
that they got a truthful representation. If that rule were 
observed, the objects would have the same apparent magni- 
tude as they would have to the eyes of a person standing on 
the spot from whence the view was photographed ; that well 
known rule was too frequently forgotten. 
February 2nd, 1865. 
J. P. Joule, LL.D., F.R.S., Vice-President of the 
Section, in the Chair. 
Mr. A. Brothers, F.R.A.S., exhibited a stereoscopic picture 
of the Blue John Mine in Derbyshire, which he had taken by 
aid of the magnesium light, giving an exposure of five minutes. 
The negative was slightly fogged, owing to the lenses not 
having been wiped when descending from the cold atmos- 
phere of the surface into the mine, which is some 3 to 400 
feet below it. The dense fumes of magnesia caused by so 
long an exposure prevented another trial. 
