386 
PROCEEDINGS OP THE SOCIETY. 
Mr. Comber said he thought this could hardly be so, because the 
heat had been taken out of the beam by passing it through water. 
Mr. Morland said he had tested what was sent to him only as 
mounting media, but, being entirely # new substances, he did not know 
their qualities, because the slides had not yet had time to harden, and 
he was not sure as to the cement used, viz. Hollis’s liquid glue, being 
able to withstand any solvent action these various media might 
have. 
Mr. Madan said that no cement could be better adapted for the pur- 
pose than a mixture of good glue (or isinglass) and treacle, such as 
opticians used for cementing carbon disulphide prisms. This should be 
applied warm, and in a day or two it would become quite hard and 
strong. He had a hollow prism with sides cemented thus, and filled 
with carbon disulphide, which had stood for at least twenty-five years 
without the slightest leak. 
Dr. Dallinger could only, in regard to the present usefulness of 
mounting media of high refracting power, give the testimony of his 
experience with slides mounted with these media. He had never mounted 
with any of them himself, but specimens had been mounted for him by 
the most competent mounters, and he had possessed and used a very 
large number of slides mounted in nearly every medium which had 
been mentioned, and he had at the present time only one slide in all 
his cabinet which was in good condition. All had changed and ruined 
most valuable specimens. Nevertheless it was most important that such 
media should be available, and it was with great pleasure he found that 
so competent a student as Mr. Madan was working upon so important 
a matter. There was hope that an efficient and useful medium of high 
refracting index might yet be found. 
The President was very glad to have heard this valuable paper. 
He had sent the materials he had received from Mr. Madan to 
Mr. Morland, who had very kindly mounted some diatoms in them so 
that he might test them. He found that the methylene di-iodide was a 
very dense substance, and the diatoms stood out well in it, but unfor- 
tunately they were injured in some way by it, so that in looking at 
them the effect was like looking at a dry slide which had gone bad 
through what was known as sweating. What it was that caused the 
structure of the diatom to melt off in this way he did not know. In 
the other materials the diatoms stood out strongly, and the structure 
was not interfered with. The quinidine was good, but it crystallised. 
He was, however, very sorry to hear that the experiments with quinidiDe 
were not a success, because the diatoms could be so beautifully seen in 
it. He had not been quite so unfortunate in liis experiences as Dr. 
Dallinger, because he had two slides which were in good condition, but 
one of these, the finest he had, was he feared, just beginning to show 
signs of crystallisation. Ail his piperine slides were still good, and 
those in Father Thompsons medium were also perfect. The first slide 
mounted in the dense arsenic medium was still perfect, but some wavy 
lines were beginning to appear. The curious thing about piperine was 
that it made the object look as if you had a bad objective ; one of the 
reasons for which was the fact that it had such a high dispersive power. 
With regard to these- co-efficients, he had worked out the comparison of 
