PROCEEDINGS OF SOCIETIES. 303 
of the real surface, and the imagination raises the object to that 
point. A diagram of the convergence of the optic axes on an 
object, before and after the interposition of the prism, will show 
that when the thin edge of the prism is turned towards the nose 
the effort made to unite the real and the refracted image is the 
same as if the vision was directed to a point more distant than the 
real object. The opposite to this takes place when the base of 
the prism is turned towards the nose. It is very possible that 
the pseudoscopic vision through prisms may haye been noticed by 
others, but I have not been able to discover any description of 
such in the works to which I have access. 
Mr. Heys said he had been asked by Mr. Dale to introduce to 
the notice of members a preparation of Canada balsam for mount- 
ing, which has the property of hardening in avery short time. It 
consisted of balsam first made perfectly solid by evaporation, and 
then dissolved in bisulphide of carbon. He found the smell a 
strong objection to its use, but the results were very satisfactory, 
the balsam on a slide becoming perfectly hard ina few hours. He 
thought, however, the dry balsam might prove to be the more im- 
portant element in the preparation, and that its solution in chlo- 
roform would probably be found to answer all practical purposes. 
Mr. Dancer exhibited many beautiful photographs of micro- 
scopic objects by Dr. Maddox. 
March 27th, 1865. 
Joun Parry, Esq., in the Chair. 
Sections of various shells were exhibited by Mr. Parry. 
An apparatus for applying pressure to the cover-glasses of 
objects freshly mounted in Canada balsam. It consisted of a 
dozen small upright pistons placed in a frame, and each furnished 
with a spiral spring coiled around the rod and pressing against 
the upper horizontal bar of the frame. 
Dr. Alcock exhibited a second time his specimens of shells of 
marine Entomostraca from the coast of Galway. He said that 
renewed examination of them and the collection of many more 
specimens had strengthened his belief that, however numerous 
their forms, they are entitled to be considered as so many distinct 
species, and he did not think that the general arguments used by 
Professor Williamson in support of an opposite opinion, at the 
meeting of February 26th, had any application to their particular 
case. In the first place, it was stated that the outer skin or shells of 
these creatures is of less value for distinction than the internal 
parts ; but Dr. Baird had described nine out of his fifteen species 
from living specimens, and yet in these his specific distinctions 
are mainly derived from the shells. Again, we were reminded 
that some of the Entomostraca are known to undergo metamor- 
phoses, and that this might probably be the case with the genera 
Cythere and Cythereis ; but Cypris, which approaches very nearly 
