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was as low as half a guinea. Those which he had seen were quite 
incapable of doing what was done by those of Zeiss, which were so true 
in centering that when tested by the light from a globule of mercury 
they showed the diffraction rings with great accuracy, but the cheap 
English glasses would not do this because they were in fault both as to 
centering and curving. Some years ago he got an ^ inch, and on testing 
it he found that the rings showed an exact representation of Saturn 
and his rings seen perpendicularly to the plane of the rings, instead 
of which it ought to have shown the rings to be all nearly the same 
in fineness. He considered that the accuracy of the rings showed the 
value of Zeiss' glasses to be tenfold greater than that of the cheap 
English glasses. All opticians would agree that to make a really 
good low-angled glass would be easier than to make a high one ; but 
then, as Mr. Crisp had said, " demand creates supply." He should 
like to mention that in 1862 he had the good luck to possess some 
fine glasses by Powell, and amongst them was an ^ inch of 70°. It 
was at that time about forty years old, and he could do with that 
glass what he had never been able to do with others. 
Mr. Wenham said that before they decided the matter they ought to 
know really what angular aperture was. He did not believe that in the 
published list of a single maker the angular aperture was actually cor- 
rect as stated. He had found that by stopping them down their perform- 
ance was improved, because otherwise a great many extraneous rays 
were admitted which caused a milkiness. He thought that a diaphragm 
stop could be adapted to these objectives with great advantage. 
Mr. Slack said he had upon the table a microscope with one of 
Zeiss' glasses, J, 48° aperture, and he had just fitted to it one of 
Ross's E eye-pieces, and it worked well. He asked if any cheap 
English objective would stand any eye-piece so deep ? 
Mr. C. Stewart said he had recently the opportunity afforded him 
of comparing Zeiss' glasses with those of Hartnack, and really the 
great working room in the case of Zeiss' was remarkable. Besides 
this, there was great flatness of field, great penetration, and a large 
amount of light. This amount of light was of great value in looking 
through opaque tissues. In making a comparison with Hartnack's 
highest powers he should say that Zeiss' was almost, if not quite, 
equal in sharpness of definition. He tried them with a slide of Top- 
ping's — of teased muscular fibre of the pig — and he thought that 
Zeiss' seemed to show over Hartnack's a rather flatter field, more 
penetration, and there was more working room ; but there might have 
been a trifle more sharpness of definition in favour of Hartnack's. 
Mr. Slack said he had felt a good deal of difficulty in bringing 
this subject before the Society because of having to mention names. 
So far as he knew, no first-class work had been lately employed upon 
any low-angled glass in this country, unless in the case of one 
specially ordered. Dr. Gray appeared to be under the impression 
that the cheap low- angled English glasses were good ; but he had 
seen many, and could only describe them as being moderately bad, 
and as far as standing any deep eye-piece was concerned they simply 
broke down altogether. A small angle, with the help of Mr. Wen- 
ham's reflex illuminator, had a great increase of resolving power. 
