384 
PROCEEDINGS OF THE SOCIETY. 
MEETING 
Held on April 20th, 1898, at 20 Hanover Square, W., 
The President, E. M. Nelson, Esq., in the Chair. 
The Minutes of the Meeting of March 16th last were read and con- 
firmed, and were signed by the President. 
The Donations to the Society (exclusive of exchanges and reprints) 
received since the last meeting were announced as follows : — 
From 
Three Slides of Botanical Sections Mr. W. T. Suffolk. 
Journal of the Board of Agriculture, vol. iv. No. 4, 1898, 8vo .. The Board. 
Report of the British Association, 1897 (London, 1898), 8vo .. Mr. F. Crisp. 
The thanks of the Society were voted to the donors. 
Mr. Xtousselet exhibited a new form of metal lamp chimney made 
with two openings, enabling white or tinted glass to be used as required 
by simply turning the lamp partly round. This chimney had been made 
to a special order by Mr. Pillischer, and happening to see it in the shop, 
he thought it might interest the meeting as a new idea in the way of 
Microscope lamp chimneys. 
The President exhibited a new monochromatic screen trough which 
had been handed to him by Mr. Curties. It was of American invention, 
and consisted of two glasses cemented to a ring. It was very simple in 
construction, and he had no doubt would be an efficient trough for a light 
filter. 
He also exhibited a dynameter, an instrument designed by Ramsden 
in 1775 for measuring the diameter of the emergent pencil of a telescope 
in order to determine the power. This was accomplished by means of a 
double image, due to the separation of the halves of a divided convex 
lens. 
The idea of a double-image micrometer by means of a divided lens is 
probably due to Savary,* but the form in which it ajipears to-day was 
originated by Dollond in 1754. Mr. Powell, to whom the dynameter had 
* Phil. Trans., 1753. 
