PROCEEDINGS OF THE SOCIETY. 
385 
been sent for adjustment, told, liim that the semi-lenses were moved by a 
differential screw. The President said that he was under the impression 
that the differential screw had been invented by his friend the Rev. 
James Campbell,* but it would appear that it was a much earlier in- 
vention. 
Mr. Ingpen said he had one of these made by Ramsden some ye\rs 
ago, but gave it away to an oculist. He knew then that it had a differential 
screw, and he tried at the time to get this introduced for other purposes, 
but was told it was too comjdicated and expensive. He also had at the 
present time a divided lens by Dollond 2^ or 2£ inches in diameter. 
The thanks of the Meeting were voted to Mr. Rousselet, Mr. Curties, 
and the President for bringing these items for exhibition. 
Mr. H. G. Madan read a paper “ On some Organic Substances of High 
Refractivity available for mounting specimens for examination under 
the Microscope.” Samples of the various substances were exhibited in 
illustration. 
He said he had sent specimens of three of them to Mr. Nelson, who 
had handed them to Mr. Morland for him to try their usefulness as 
media for mounting diatoms, and he hoped to hear something as to the 
result from Mr. Morland that evening. 
Mr. T. C. White said they were greatly indebted to Mr. Madan for 
the very practical paper he had read. He should like to ask if these 
media were suitable for histological work ; because, although they might 
do admirably for diatoms or osseous tissue, they might not answer for 
softer substances. 
Mr. Madan said that one or two were as harmless as Canada balsam ; 
some others he could hardly say as much for ; but piperine and quinidine 
had been tried and found harmless, and monobromonaphthalene and 
phenylthiocarbimide, being neutral substances, were not likely to injure 
even delicate tissues. Time alone could prove the entire satisfactoriness 
of these media ; but he was inclined to think that the mixture of meta - 
cinnamene and phenylthiocarbimide mentioned in the paper approached 
most nearly to balsam in permanency and neutrality, while it had a 
much higher refractivity than the latter. It did not, however, harden 
so completely, hence it should be treated like Farrant’s medium. 
Mr. T. Comber said that their old friend Dr. Gray had tried experi- 
ments with some of the substances mentioned, and gave him some slides 
mounted with piperine, &c., which were very good at the time, but did 
not last, except one which was put up in a mixture of piperine and 
styrax. The styrax was heated, and whilst in this condition crystals 
of piperine were put into it with the object of increasing the refractive 
index, and this slide showed no sign of change whatever. He had tried 
some of these mixtures as media for objects which he wanted to photo- 
graph in sunlight, but found they were not particularly adapted for 
this, because they were affected by the sunlight and rendered dark in 
colour. 
Mr. Madan thought this might be due rather to the heat than to the 
light of the sun. 
* Described at the Q. M. C., 1886. 
2 D 2 
