270 
PBOCEEDINGS OF SOCIETIES. 
Dr. Matthews inquired what would be the result if the eye-piece 
were entirely non-achromatic ? 
The President said he had never tried how that would be, but he 
was sure they would get the same ultimate results, and that the spec- 
trum would be the same, though not all in focus at the same time. 
Mr. Slack read a paper " On Angle of Aperture, and its Eolations 
to Surface Markings and Accurate Vision." 
The President, in proposing a vote of thanks to Mr. Slack for his 
paper, expressed his entire concurrence with the remarks it con- 
tained ; he had occasion some time ago to examine specimens of rocks, 
and very soon came to the conclusion that a large angular aperture 
was far inferior to a small one, and that the latter was immensely 
superior in penetrating power. He fully agreed with Mr. Slack that 
they ought to have object-glasses made on these principles, and that 
they ought to encourage opticians to make them in order that persons 
might have what was really of the greatest use for research, instead of 
what was only of value for a few special objects. 
Mr. Wenham said he agreed with Mr. Slack in the general purport 
of his observations, and he should himself always choose the glass 
which did the work required with the smallest angle. As a matter of 
fact the value of aperture was scarcely appreciable between 150° and 
170°, and as regarded the extreme marginal rays, generally, he 
believed they did more harm than good on account of the quantity of 
false light which they admitted. He did not really believe that large 
apertures did the slightest good, and felt sure that with an angle of 
90° they might do all that was required. 
Mr. Stephenson said that through the kindness of Herr Zeiss he 
had been able to examine a series of his glasses, and could quite 
confirm all that Mr. Slack had said as to their remarkable working. 
Persons were much mistaken in supposing that a diatom was the 
proper object with which to test the quality of an objective, and he 
ventured to say that at their late soiree many people thought that 
Surirella gemma was exhibited by Mr. Slack for the purpose of 
showing the markings, whereas it was shown only to prove how well 
so extremely low an angle was capable of performing. 
Mr. Frank Crisp thought it was of no use to abuse English opti- 
cians for making objectives of high angle ; people would have them ; 
and the question became one merely of demand and supply. It was 
really the fault of microscopists, and it was they who required to be 
educated on the matter. 
Dr. Gray, in reply to a question from Mr. Slack, said he was 
chiefly struck with the great distance through which this glass would 
work. He thought, however, that there were a great many English 
opticians who made cheap educational glasses which would do the 
same thing as this one. 
Mr. Slack thought that no one had yet produced one like it with 
an angle of 48°, nor did he know of any like the one with which he 
showed Surirella gemma at the soiree, of 68°. 
Dr. Pigott said he had been much surprised at the price of some 
of these low-angled English glasses ; they were, he believed, made at 
a distance, and ho had heard that the original cost of some of them 
