268 



The application of a concave achromatic lens to the wired micro- 

 meter of a telescope, arose out of the series of trials that were made 

 for the purpose of correcting the aberrations of the eye-glasses applied 

 to the telescope constructed by the author for the Royal Society, with 

 a fluid-correcting lens, on the plan suggested by Professor Barlow. 

 The concave lens, being interposed between the object-glass and the 

 eye-glass, and being at the same time achromatic, combines the ad- 

 vantages of doubling the magnifying power, without a corresponding 

 diminution of light, and without altering the apparent distances of 

 the threads of the micrometer. The results of the trials made with 

 telescopes to which this addition was made, are given in a letter to 

 the author from the Rev. W. R. Dawes, of Orniskirk j from which it 

 appears that Mr. Dollond's method was attended with complete suc- 

 cess. Mr. Dawes states, that, in order to put its illuminating power 

 to a severe test, he had examined with this instrument the satellites 

 of Saturn and the minute companion of k Geminorum, but could dis- 

 cover no decided difference in the apparent brightness of the former, 

 allowance being made for the difference in the power employed j and 

 the latter star was seen quite as distinctly with a much smaller power. 



Extracts are subjoined from a letter of Professor Barlow's to the 

 author, containing formula? for the construction of the lens. 



March 6th, 1833. 



MARK ISAMBARD BRUNEL, Esq., Vice-President, in the Chair. 



The reading of a paper, entitled, " On the Structure and Functions 

 of tubular and cellular Polypi, and of Ascidiae." By Joseph Jackson 

 Lister, Esq., F.R.S. — was commenced. 



March 13th, 1833. 



JOHN WILLIAM LUBBOCK, Esq. M.A., V.P. and Treasurer, 

 in the Chair. 



The reading of Mr. Lister's paper was resumed and concluded. 



This paper contains the account of a great number of observations 

 made by the author during the last summer, while he was at the 

 southern coast of England, on several species of Sertularice, Plumu- 

 laricEy TubularicB, Campanularice, Flustrce, and other polypiferous zoo- 

 phytes, and also on various Ascidice. Each specimen was placed for 

 examination in a glass trough with parallel sides, before the large 

 achromatic microscope of the author, directed horizontally; and care 

 was taken to change the sea- water frequently, which was done by 

 means of two syphons, the one supplying fresh water, while the other 

 carried off the old ; a plan which succeeded in keeping the animals in 

 perfect health and vigour. The drawings which were taken of the 



