PROCEEDINGS OF THE SOCIETY. 
275 
co-operation of Mr. F. Justen, a candidate nominated for the Fellowship 
of the Society that evening. 
Prof. Bell said he had asked the opinion of Mr. Davis (of the 
British Museum) upon this subject, and he told him that it was almost 
impossible to get it clear anywhere. 
Mr. Crisp thought that if it was a fact that this crystal was used as 
suggested, it seemed as if something else would do as well, because, 
when Mr. Powell produced his lens, it was said to be nearly, if not 
quite, equal to those produced by Zeiss. 
Mr. Powell said that their lenses were not made of it, but he felt 
equally sure that the first apochromatic lenses sent to England by Zeiss 
were not altogether made of Schott’s glass as was supposed. 
Mr. Mayall said the matter referred to by Mr. Crisp needed explana- 
tion. There was no doubt that when Mr. Powell brought out what he 
termed his “apochromatic” objective, and it was compared with the 
Zeiss apochromatic, the opinions of experts were balanced as to their 
comparative merits. The estimations were then made by the eye only 
— by the images seen in the Microscope. Since then, however, it had 
been found that the production of photomicrographs by the rival 
objectives was a still more searching test — a test that could not be 
neglected when once fairly tried. Judged, then, by the photomicro- 
graphic test, the Zeiss apochromatic objectives proved superior, although 
when compared by the eye only the rival lenses showed but minute 
difierences — differences in the earliest trials that were slightly in favour 
of Mr. Powell’s work. With reference to Mr. Powell’s remarks as to 
the various kinds of glass employed in the apochromatic constructions 
by Zeiss, he (Mr. Mayall) thought Mr. Powell’s criticism was probably 
accurate. At the same time he did not think there was any obligation 
on the part of Zeiss to explain what materials he employed. If Prof. 
Abbe chose to communicate the fact that fluorite was one of the elements 
so employed, that was a matter of great scientific interest, and if other 
opticians availed themselves of the use of the mineral the construction 
of Microscope objectives would doubtless make great progress. 
Mr. Nelson said he saw several glasses of foreign construction very 
shortly after Zeiss’s were introduced ; they were made as copies of Zeiss’s 
and were apochromatic, and although they were of great excellence, they 
did not appear to be so well corrected as Zeiss’s. He had no doubt that 
certain German opticians had found out the secret of Zeiss’s apochromatic 
constructions, though their workmanship was not equal to Zeiss’s. 
Mr. T. F. Smith said it had been known to him for some time that 
some mineral had been used in the construction of these lenses, which 
gave results not previously obtained with glass. 
Mr. Mayall said it might be advisable to correct an error by Dr. 
Pelletan in his description of the new lens ; he mentioned the price as 
10,000 francs or 400Z. It appeared that the fact was, as suggested by 
Mr. Crisp at the time, an extra zero had been added ; the actual price 
was 1000 francs or 40Z. 
Mr. C. H. Gill read a paper “ On some methods of preparing Diatoms 
so as to clearly exhibit the nature of their markings.” He illustrated 
the subject with numerous photomicrographs. 
