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PROCEEDINGS OF THE SOCIETY. 
matter. He did not say that a cross was produced, but “ what looked 
like a cross ” — not being an image, but an effect caused by the inter- 
section of points. 
Mr. Nelson thought Mr. Crisp was mistaken in his description of 
what was done by Dr. Matthews, for although he showed crosses in the 
case of some of the coarse diatoms, he did not produce them with 
P. formosum, but he did produce a bar which was the image of a pin. 
It should be remembered that at that time it was more difficult to show 
these things than with their present apparatus. It was always exceed- 
ingly difficult to show the image in the markings, because by the time 
they reached the image the markings were out of focus, but they could 
sometimes show a very much out-of-focus hole with some sort of image 
got out of it. He remembered that at a microscopical soiree some one 
showed the seconds hand of a watch in the eye of a beetle, but it was 
found impossible at the same time to show the hexagonal holes. He 
thought they would also be unable to show it in photography, because 
the holes would have all run together by the time the crosses were 
focused. Possibly the new lens of 1*6 N.A. might help them. 
A note was read from Dr. van Heurck correcting an error in his 
recent communication to the Society relating to the structure of diatoms. 
An enlarged photograph of a photomicrograph of P. angulatuin by 
Dr. van Heurck was also handed round for inspection. 
Mr. Crisp thought that for P. angulatum it was a remarkable photo- 
graph. 
Mr. Mayall read a translation of an article * by Prof. E. Abbe in the 
‘ Zeitschrift fiir Instrumentenkunde,’ of January, relating to the use of 
fluorite for optical purposes, in which it appeared that the special 
qualities of the new apochromatic objectives were due to the employ- 
ment of fluorite lenses in their construction. This mineral had lower 
refractive and dispersive indices than any optical glass hitherto produced, 
and its introduction as a new element in the construction of Microscope 
objectives enabled the optician to reduce the spherical and chromatic 
aberrations much below the point previously reached in achromatic 
combinations of the usual construction. Following upon the publication 
of Prof. Abbe’s note, Mr. Mayall said, the essential secret of the apo- 
chromatic formula appeared to be disclosed, and he hoped the English 
opticians would soon recover the ground lost through their neglect to 
discover the fact of the employment of fluorite in Zeiss’s new lenses. 
The Society were of course most desirous of promoting optical improve- 
ments, and as it appeared that fluorite in crystals of the requisite size 
and clearness seemed hardly obtainable in Europe, it was important that 
other sources should be found. He trusted that those Fellows who had 
correspondents abroad, where there was any probability of obtaining 
such fluorite, would not fail to engage them to inquire as to the possi- 
bility of discovering a supply of the mineral, and would bring the matter 
before the Society, should their efforts prove successful. Applicatiious 
were already on the way to the Brazils, Chili, and Peru, thanks tot the 
* To be published in the next number of the Journal. | 
