Monthly Mi ical 
Sr A ae er PROCEEDINGS OF SOCIETIES. 289 
deep-sea dredgings made by Lieut. Baker, R.N., off the coast of 
Malabar. 
The Chairman announced that a communication had been received 
from Mr. Hankey, of Chicago, recently elected an Hon. Fellow of the 
Society, offering as a token of his interest in the Society a prize of five 
guineas for the best essay “On the Microscope in Education.” The 
thanks of the meeting were unanimously given to Mr. Hankey for his 
offer, which, the Chairman stated, would be considered by the Council, 
and the conditions under which the essay was to be written made 
known as soon as their decision was made. 
It was stated that a paper by Mr. J. Beck, “On the Mode of 
Obtaining Definition of the Structure of the Scales of Thysanure,” 
would, with his consent, be taken as read, printed in the Journal, and 
discussed at the next meeting, in conjunction with one on a similar 
subject by Mr. McIntire. 
Mr. W.S. Kent then read, in brief abstract, a paper “On the 
Coralliospongiz, or Anastomosing Sponges.” 
The Chairman, on thanking Mr. Kent on behalf of the meeting, 
expressed a hope that the results of his deep-sea dredging expedition 
would be published in eaxtenso in the ‘'Transactions’ of the Society, 
and be made the subject of a discussion at some future meeting of the 
Society. 
Dr. G. W. Royston-Pigott then gave a viva voce exposition of a paper 
“On Aplanatic Illumination and Aplanatic Definition.” 
Mr. Browning inquired whether, in regard to the size of spurious 
disks, Dr. Pigott did not think it was dependent upon the diameter of 
the object-glass, and that consequently the larger the angular aperture 
of the object-glass, the smaller the disk would be. He wished also to 
express his admiration of the ingenuity of the experiments which 
Dr. Pigott had made on the subject of his paper. 
Dr. Pigott stated in reply that he had in telescopes observed the 
effects mentioned by Mr. Browning, and that he had found the size of 
the disk diminish as the aperture of the object-glass increased. But 
the experiments upon spurious disks were attended with some difficulty, 
for the laws as to these disks in the microscope differed from those 
which obtain in the telescope, since the microscope defines under 
totally different conditions. He believed he was correct in stating that 
no telescope ever got an angle of aperture of more than 7°, some 
indeed had only 5°; whereas in the microscope you might have an 
object-glass with 170° of aperture. 
Dr. Pigott also alluded to definition under the binocular microscope, 
and stated that Dr. Carpenter had told him that he was unable to 
define the spherical form of Polycystine until he requested Messrs. 
Powell and Lealand to make for him a }-inch object-glass of only 40° 
aperture; larger apertures failed. 
The Chairman said he should like to have some explanation of the 
construction of the aplanatic searcher. 
Dr. Pigott replied that the aplanatic searcher consisted of a pair 
of achromatic lenses somewhat over-corrected. There was some 
subtilty about its application, and just in this way: If the lenses were 
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