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PROCEEDINGS OF THE SOCIETY. 
minimum, and is 27 per cent, less than that of Sir John Herschel’s. 
This doublet differs from Herschel’s both in the ratio of the radii of the 
meniscus and also in the ratio of the foci of the two lenses ; the only 
point of similarity is in the first lens, which is “ crossed.” * 
Permit me again to urge every microscopist to try for himself the 
following simple experiments. Project the image of the flat of a lamp- 
flame on a piece of white card with a plano-convex lens (such as the 
field-lens of a Huyghenian eye-piece), using first the convex side and 
then the plane side towards the card, the lamp being about 6 ft. from 
the lens. When the convex side of the lens is towards the card, and the 
lamp-flame as sharply focused as possible, a circular halo of misty light 
will be seen to surround the lamp-flame ; but when the plane side of the 
lens is facing the card this circular halo of misty light will be reduced, 
and the brightness of the image of the flame will be much increased, 
because a portion of the light which before was scattered and formed a 
part of the misty halo is now condensed in the image of the lamp-flame. 
If the lens were strictly aplanatic there would be no misty halo, all the 
light being concentrated in the image of the lamp-flame, and the image 
of the flame would be of maximum brightness under the given conditions. 
In brief, the diameter of the halo of misty light is a measure of the 
spherical aberration. If one of my condensers, of the form of minimum 
aberration for two pianos, be compared in the above manner with an 
ordinary single bull’s-eye of the same focus, the diameter of the misty 
halo will be found reduced to a radius of about 1 /5 in., but with this 
new condenser there is a further reduction, so that the radius of the misty 
halo measures only 1/20 in. These experiments are most instructive 
and form a good training for critical work with the Microscope, because 
the brightness or the mistiness of the Microscope image is an asso- 
ciated phenomenon. 
The President moved the thanks of the Society to Mr. Nelson for 
this communication aud exhibit, which struck him as calling attention 
to a matter of very considerable practical service. Many microscopists 
took a great deal of trouble with their Microscopes in order to get the 
best results, but did not take enough with their condenser with which 
the Microscope was supplied with light. It was of course very important, 
if the best definition was required, to take care that light of the best 
possible kind was supplied in the most perfect manner which could be 
attained ; and he thought that those who had seen this condenser and 
heard Mr. Nelson’s description of it would agree that it provided the 
means of obtaining just what was wanted. 
The thanks of the meeting were unanimously voted to Mr. Nelson. 
The President said that they had a communication from Mr. A. W. 
Waters which would no doubt be found of considerable value, as the 
author was probably the best English authority on the Polyzoa now 
living. Unfortunately the state of Mr. Waters’ health obliged him to 
reside at Davos Dorf, but Mr. Karop had kindly undertaken to read the 
paper to the meeting. 
Mr. Waters’ paper on the ‘ Interzooecial Communication in Flustridse 
* See Journ. Quekett Micr. Club, April 1896, pp. 197-205 
