PROCEEDINGS OF SOCIETIES. 175 



in preserving them. It would be excessively important to ascertain 

 if it was really true that there was a ganglionic cord. It would 

 certainly be very exceptional to find that there was really a chain of 

 gangli lying all along the tail of the animal. Usually in MoUusca 

 they are divided into three portions — one set being near the mouth, 

 another (the pedal) near the ventral portion, and another (splanchnic) 

 near the posterior extremity ; but here it would seem as if they had 

 almost a repetition of what was found in much higher organisms, and 

 seemed more like those of insects. 



Mr. Sanders said that he had used a monocular microscope for his 

 observations. The use of picric acid seemed a very good suggestion, 

 and he should like very much to know the strength which was 

 recommended for the purpose. He did not see the ganglionic cord; 

 he was not aware of its having been mentioned, so that he did not look 

 particularly for it. 



Mr. Chas. Stewart said he did not know exactly the strength of 

 the picric acid ; it was usually mixed according to colour, but he 

 would try to ascertain its actual weight. It had been found to answer 

 exceedingly well, with the slight disadvantage of staining. 



A paper by Dr. Royston-Pigott, F.R.S., entitled " A Note on the 

 Verification of Structm'e by the Motion of Compressed Fluid," was 

 read by the Secretary ; and was followed by another paper by the same 

 author, " A Note on the President's Remarks on Dr. Pigott's Searcher 

 for Aplanatic Images." 



Dr. Pigott then proceeded to give further explanations. He 

 thought he ought also to have added to the paper on the ' Searcher ' 

 that the action of the screw-collar was one which was not sufficiently 

 studied in the present day. That in eifect it either enlarged or 

 diminished the aperture of the objective. If the lenses were separated 

 it diminished the aperture, and if they were closed it increased it. By 

 closing them they shortened the focal distance and increased the 

 power of the objective. If gentlemen would just take the trouble to 

 do so they would find that by wholly closing the lenses they would 

 give the objective its greatest focal power. It was a matter worth 

 studying, because the whole question of aberration was involved in 

 that simple fact. The whole question turned upon that one thing, 

 the increase or diminution of aperture of the pencil passing through 

 the back lenses by the correction of the screw-collar.* Then there 



* Mr. Tolles' ith, lent me by Mr. Crisp, possesses extremely fine qualities. 

 But if a Kellner eye-piece be used with a very large field, objects very much out 

 of the centre are obscured by residuary aberration. Mr. Browning's achromatic 

 eye-pieces, as suggested by Rev. Mr. Webb, cut otf so much of the field that only 

 a very small portion is seen at once. The definition of Tolles' glass witlj such a 

 limited field eye-piece is superb. Bad telescopes are often cured of residuary 

 aberration by contracting the aperture. I wish particularly to call attention to 

 the little Instrument called the aberrameter, for instantly reducing by a variable 

 stop the aperture of object-glasses (made for me on the iris principle by Messrs. 

 Beck). The aberration is thus instantly controlled by gradually reducing the 

 aperture. That of any simple lens is well known to be in proportion to the square 

 of the breadth of tiie pencil admitted through it. Of course the angular aperture 

 is reduced by a stop placed at the back of an objective. It is also reduced by 

 separating the lenses of an adjustable screw-collar object-glass. Mr. Tolles' lens 



