40 
PROCEEDINGS OF SOCIETIES. 
Mr. Crisp said lie had tried it with a ~tli inch having the back cut 
down very flat so tliat it could come very close to the hack lens. 
Mr. Wenham hoped some ingenious member would invent some 
form of diaphragm to suit it. 
The President thought it might be possible to construct an iris 
diaphragm of a number of thin strips of metal in such a way that it 
could he expanded or contracted. 
Mr. Wenham had thought of this plan, and also of making an 
aluminium tube with a collar to run up and down the case, hut the 
difficulty then was to get at it to work it when in position. 
Mr. Slack asked if it would he difficult to make a kind of rotating 
diaphragm for the purpose. 
Mr. Wenham said that if it became a question of cutting through 
the object-glass, he would rather use a slide than anything to rotate. 
Mr. Slack said that in the case of Eupodiscus Argus the surface 
was too much curved for any high-angled glass to show the object 
properly, hut w r ith a low angle the piece that came into focus was 
shown perfectly. Without penetration it was not possible to get a 
projier idea of the structure. 
Dr. Matthews said that he had very little to report concerning the 
diatomaceous earth committed to him at the last meeting except a 
series of failures. He had boiled some down in nitric acid, but still it 
retained its shape, and when be crushed it the diatoms were also 
broken. He gave some to Mr. Topping, and he seemed to he able to 
do it perfectly, and had mounted a specimen. However, he was unable 
himself to make out its species, and had therefore handed it to Mr. 
Hailes, who in turn being also puzzled had sent it to Mr. Kitton, who 
was himself uncertain as to what species of Orthosira or Melosira was 
present. The specimen was exhibited under a microscope in the room. 
Mr. Slack had also looked through the collection sent by Mr. 
Hanks, and found amongst it many interesting additions to their slides 
of minerals. There was also a very fine polychroic object — sesqui- 
oxide of chromium. 
Mr. Wenham asked Dr. Matthews if his difficulty arose from the 
silicious character of the earth, as in that case he thought if it were 
boiled in a weak solution of soda the silicious material would give 
way. 
Dr. Matthews said he had tried soda and potash three times, and 
also repeated boilings in nitric acid, and had boiled it to dryness, hut 
still unable to succeed. 
The proceedings were then adjourned until October. 
Donations to the Library since May 5, 1875 : 
From 
Nature. Weekly The Editor. 
Atkenreum. Weekly Ditto. 
Society of Arts Journal. Weekly Society. 
Journal of tlie Linnean Society. No. 59 Ditto. 
Journal of the Geological Society. No. 122 Ditto. 
Monthly Notices, &c., of the Royal Society of Tasmania Ditto. 
Walter W. Reeves, 
Assist. -Secretary. 
