PROCEEDINGS OF THE SOCIETY- 
349 
added, and this rendered it very effective and useful when illuminated 
by daylight, or by the flame of a paraffin lamp. Unfortunately, it was 
of little use in that room, as the incandescent filament of an electric 
lamp was quite unsuitable for this hind of illumination. 
The other item which he had brought to show them was a new Micro- 
scope lens, viz. a semi-apochromatic 1/10 of 1*3 N.A. by Leitz corrected 
for the long tube. The optical index of this lens was 13, against 10 of 
the old semi-apochromatic 1/12, and 17 of Zeiss’ apocliromatic 1/8 of 
1-4 N.A. 
He said that, in looking back upon the past, it will be readily con- 
ceded that the greatest revolution in the history of the Microscope was 
caused by the introduction of achromatism ; but if the period was limited 
to the recent past, it might be said that a new epoch began with Prof. 
Abbe’s invention of apochromatism. His own opinion, however, was that 
the production of the new Jena glasses which enabled semi-apochromatics 
to be constructed, wos of equal, if not greater, importance to microscop- 
ists than the introduction of apochromatism. 
Apochromatism under present conditions required the use of fluorite, 
a rare mineral, and one difficult to work, consequently apochromatics 
were expensive ; but here we had a lens before us, the optical qualities 
of which were only slightly below those of the best apochromatics in 
existence, and yet its price was SI. 15 s. 
Of course the resolving power of this lens was less than that of the 1*4 
apochromat in the proportion of 13 to 14, but he doubted whether there 
was any ordinary test-object which a 1 *4 apochromat would resolve, that 
this objective would not resolve also. The introduction of such an ex- 
cellently corrected semi-apochromat encouraged him in the hope that 
before very long he would be able to announce to them that the word 
“ semi ” might be omitted from the title of this class of objective. 
He had given this lens an exhaustive testing, and there were only 
one or two excessively difficult secondary diatomic structures in his pos- 
session which it failed to resolve. 
They could remember that a little time back he announced the dis- 
covery of a difficult structure on the hoop of a Navicula major ; that 
structure, which was so difficult with an apochromat of 1*4 N.A., was 
strongly shown by this lens. This indicates not only the advance made 
by the lens itself, but also the improvement in the methods of using 
modern wide-angled oil immersions. 
Mr. Vezey said that it had frequently happened at the last meeting 
of the session that they had no paper upon the Agenda. This was the 
case on the present occasion ; but they had upon the table for exhibition 
a large number of specimens of palates of Mollusca selected from the 
collection presented through Mr. Rousselet at the last meeting ; these 
being shown, as the President had already mentioned, under Micro- 
scopes kindly provided for the occasion by Messrs. Watson and Son. 
He also intimated that the Rooms of the Society would be closed from 
Monday to Wednesday, June 21st to 23rd inclusive, and that they would 
be closed for the usual vacation from August 13th to September 13th. 
2 B 
1897 
