136 
PROCEEDINGS OF THE SOCIETY. 
new scale of powers, and on the other a scale of diopters, the lines in 
all cases being brought to the edge so that they could readily be com- 
pared with those on the adjoining sides. It would be found useful not 
only in enabling the power of a simple Ions to be read off at once by 
measuring its focal distance, but the magnifying power of a combination 
of lenses could also be ascertained very simply by the measurement of 
each. 
Mr. C. Beck enquired if the scale of powers represented the re- 
ciprocals or the optical powers? With reference to the tube-length, he 
thought it was stated in the Continental catalogues that the lenses were 
corrected in all cases for one tube-length. 
Mr. Nelson said the scale was that of the optical power, which was 
the same thing as the reciprocal of the focus. He knew it was generally 
understood that the foreign objectives were corrected for a tube-length 
of 6 in. ; this was not the case. They might have been so corrected as 
for adjustment upon an uncovered object, but when used upon covered 
objects this was found to be entirely delusive. Some time since he was 
examining a rotifer with one of these objectives, but neither he nor 
Mr. Kousselet were able to see the cilia with it, and he found it was 
incapable of showing them because it had been corrected for a shorter 
distance than 6 in. ; but when they reduced the tube-length to 5 in. it 
showed them perfectly. In practice he found that the 6 in. was a myth, 
and that the actual distance for these objectives might be anything from 
5 in. to 7 in. 
Mr. J. E. Ingpen said Professor Abbe decided that his 10-in. tube- 
lengths should be measured from the back of the combination to the 
field front, and he always understood that the 6-in. tube-lengths were 
to be measured in the same way. It was, he thought, a great pity that 
he could not have been induced to arrange to assimilate the two. 
The President said their thanks were due to Mr. Nelson for his 
exhibits and also for the very clear way in which he had explained them. 
The arrangement for rotating the mirror was a matter of some import- 
ance as providing an effective means of getting rid of the multiple images 
which were sometimes a source of very great inconvenience. 
The President said they were that evening honoured by the presence 
of Dr. H. C. Sorby, whom they were extremely pleased to welcome as one 
who had done such good service to the Society in years gone by. He 
was happy to say that Dr. Sorby had kindly consented to give them 
some account of the successful methods he had adopted for mounting 
and exhibiting many of the very beautiful and delicate marine organisms, 
and he would therefore call upon him to do so. 
Dr. H. C. Sorby said he had not the remotest idea when he came to 
the meeting that he should be asked to say a word, but as Dr. Dallinger 
had told him that there was very little upon the Agenda paper and had 
asked him to say something about the methods he had adopted to pre- 
serve some of the more delicate marine organisms, he had consented to 
do so, although he felt that, not having the specimens with him, there was 
some fear that his description of them would not be so intelligible as 
might otherwise have been the case. 
