66 
Sussex, especially amongst the young people. A system of exchanges 
between this and other-museums had already been adopted, and he 
should be glad if collectors would think of the Museum as well as of 
their private cabinets, as any specimens would be useful for exchange. 
MARCH 26TH. 
MICROSCOPICAL MEETING.—MR. T. H. HENNAH 
ON ILLUMINATION. 
Far more depended on the proper illumination than on the objec- 
tives ; inferior ones being made to show a good deal, while good ones 
without it failed to show anything like a proper performance. If 
objects were illuminated by transmitted light, parallel rays were very 
important, no matter what the illuminating apparatus, whether mirror, 
condenser, or prism. By carefully attending to this, almost any 
structure might be brought out by the mirror alone, without having the 
expensive substage appliances often recommended. 
The angle of illumination should not exceed 90°, otherwise, instead 
of clearness, there would be indistinctness and confusion. No matter 
what the form of substage appliance, the light should be exactly 
focussed on the object, a point to which sufficient attention was not 
always paid. For thin and lined objects, or for diatoms, oblique rays 
were essential, but, for thick ones, direct rays were necessary, 
There were several different methods of obtaining direct or 
oblique illumination by substage apparatus, such as the ordinary and 
flat mirror, the Nachet and Amici prism, the Reade’s prisms, and the 
various forms of Achromatic Condenser ; the action and mode of using 
each was described, preference being accorded, when the substage 
admitted of its application, to the Amici prism. 
The different methods for illuminating opaque objects and for 
obtaining dark ground illumination, were the bull’s-eye condenser, the 
Leiberkuhn, Parabolic illuminator, Wenham’s parabaloid, &c. 
After a vote of thanks to Mr. Hennah, the Meeting became a 
Conversazione, when interesting objects were exhibited by Messrs. 
Hennah, Haselwood, W. H. Smith, and Dr. Hallifax, the last-named 
