159 



the unusual circumstances relative to this comet, which have involved 

 the computation of its elements in difficulties not often met with, 

 and which induce him to request Dr. Wollaston to lay the method 

 by which he proceeded before the Royal Society. 



On the Electrical Phenomena exhibited in Vacua. By Sir Humphry 

 Davy, Bart. P.R.S. Read December 20, 1821. [Phil. Trans. 

 1822, p. 64.] 



The relations of electricity to space, as nearly void of matter as it 

 can be made on the earth's surface, are connected with many im- 

 portant queries bearing upon the nature of heat, light, electricity, and 

 magnetism. 



The vacuum used by Sir Humphry Davy was that above the mer- 

 cury in the barometer tube, and a more perfect one produced in the 

 same way by fused tin ; the former he found always permeable to 

 electricity, but the colour and intensity of the light in traversing the 

 mercurial atmosphere was remarkably affected by its temperature ; 

 it became green and vivid when the tube was heated, and was scarcely 

 perceptible in a very dark room, when it was cooled to ; which 

 phenomena, as well as some others described by the author, are refe- 

 rable to the varying density of the mercurial vapour. The admission 

 of a little air rendered the light blue, and improved the conducting 

 power of the medium. The most perfect vacuum that could be ob- 

 tained above fused tin, was also permeable to electricity ; but the 

 light was yellow and exceedingly pale, and only slightly increased 

 by heat. Electric and magnetic repulsions and attractions took place 

 in the mercurial vacuum, as in air ; a circumstance which shows, 

 says Sir Humphry, that they are not dependent upon elastic ponder- 

 able matter, and point them out as primary causes of other electrical 

 phenomena. 



From the aggregate results of his researches, the author thinks it 

 evident that the light, and probably the heat, generated in electrical 

 experiments, depend principally upon some properties or substances 

 belonging to the ponderable matter through which it passes, and 

 they render it probable that it is entirely owing to this source. 



Croonian Lecture. On the Anatomical Structure of the Eye ; illus- 

 trated by Microscopical Drawings, executed by F. Bauer, Esq. By 

 Sir Everard Home, Bart. V.P.R.S. Read November 15, 1821. 

 [Phil. Trans. 1822,^.76.] 



Having ascertained, by the aid of Mr. Bauer's microscopical obser- 

 vations, that neither the marsupium nor the ciliary processes are 

 muscular, and therefore inadequate to those adjustments of the cry- 

 stalline lens requisite for distinct vision ; and that the structure of the 

 choroid coat is also membranous, the author turned his attention 

 to the structure of the iris, which in the human eye resembles that 

 of the quadruped developed by Mr. Maunoir in his Treatise on the 



