PROCEEDINGS OF THE SOCIETY. 277 



always found a dark ground produced by a condenser and stop preferable 

 to that obtained by any paraboloid or similar apparatus. He attributed 

 this to the more perfect concentration of rays, that is the aplanatism of 

 the condenser. 



A short time ago he also pointed out that there was considerable 

 difficulty in the employment of any of the condensers he had owing to 

 the different thicknesses of the glass slips now in use ; building them up 

 by means of slip equalizers being an intolerable nuisance ; and he asked 

 Mr. Curties if he would undertake to construct a new illuminator if he 

 worked out the formula. This Mr. Curties consented to do, and the one 

 exhibited that evening had just been made in strict accordance with the 

 formula Mr. Nelson had sent him. He supplied Mr. Nelson with the 

 measurements of a large number of slips as now used by mounters, and 

 the condenser was constructed so that it would work with those ranging 

 from 0*8 to 1 • 2 mm. He had tested it upon diatoms and bacteria, and 

 found that it gave a perfectly dark field with a Zeiss Apo. 4 mm. of 

 0*95 N. A. Bacilli were brightly shown with great sharpness on a 

 velvety dark ground, and they were sufficiently well illuminated for 

 deep eye-pieces to be used. It was fitted with a fixed central stop, but 

 if wished a slot could also be used, and this turned the apparatus into a 

 very oblique illuminator suitable for resolving the finest ruled bands or 

 diatomic striae : Amphipleura pellucida was quite easily resolved with a 

 T Vth of N.A. 1*30. A small dot had been placed upon the front lens 

 for the purpose of centring the condenser to the optical axis of the 

 objective ; without some such plan there would be trouble in getting it 

 centred. It was fitted with the Society's screw, so that it would readily 

 fit the usual substage adapters. 



The President said that the lenses did not seem to him to be 

 achromatic. 



Mr. Curties replied that they were not, but merely two single lenses. 



The thanks of the Society were accorded to Mr. Curties for his com- 

 mu nication . 



The President then called upon Messrs. Heron-Allen and Earland 

 for their paper, and also referred to the microscopical preparations of 

 Foraminifera exhibited in the room. 



Mr. Heron-Allen in his preliminary remarks said that some pott had 

 said that " the best of friends must part." He was now in the hard 

 position of one about to take a formal leave of the Fellows of the 

 Society prior to his embarking with Mr. Earland upon some years of 

 research work upon the 'Goldseeker' dredgings from the North Sea, and 

 it was his desire to express their joint thanks for all the favours they 

 had received in the past. There were several, he felt, to whom their 

 most heartfelt thanks were due : first of all he wished to thank the 

 Council for always having allowed them practically unlimited space in 

 the Journal for the publication of their papers ; then the Treasurer 

 for his liberality in the matter of printing their plates. He considered 

 that more than an ordinarily wide latitude had been allowed them in the 

 matter of illustrations ; he had often after handing in the MS. and 



