46 Mr Brown 07t Active Molecules 



tially in its statements with the edition of 1819, and which I have 

 never seen, and of the existence of which I was ignorant when 

 I pubhshed my pamphlet. 



From the edition of 1828, which I have but lately met with, it 

 appears that Mr Bywater employed a compound microscope of 

 the construction called Culpepper's, that the object was examined 

 in a bright sunshine, and the light from the mirror thrown so 

 obliquely on the stage as to give a blue colour to the infu- 

 sion. 



The first experiment I here subjoin in his own words : — 



" A small portion of flour must bo placed on a slip of glass, 

 and mixed with a drop of water, then instantly applied to the 

 microscope ; and if stin-ed and viewed by a bright sun, as already 

 described, it will appear evidently filled with innumerable small 

 linear bodies, writhing and twisting about with extreme activity." 



Similar bodies, and equally in motion, were obtained from 

 animal and vegetable tissues, from vegetable mould, from sand- 

 stone, after being made red hot, from coal, ashes, and other in- 

 organic bodies. 



I believe that in thus stating the manner in which Mr Bywa- 

 ter's experiments were conducted, I have enabled microscopical 

 observers to judge of the extent and kind of optical illusion to 

 which he was liable, and of which he does not seem to have been 

 aware. I have only to add, that it is not here a question of 

 priority ; for if his observations are to be depended on, mine 

 must be entirely set aside. 



On the Tripang; or B'lcho dc Mar, or Sea-Sliig of India — the 



Holothuria tuhulosa of Naturalists. Jiy Chaiiles Collier, 



formerly Staff-Surgeon in Ceylon, now Inspector of Hospitals 



in the Mauritius. Communicated by Sir James Macgrigou. 



With a Plate. 



Having had many opportunities of observing this species, 



and thinking that, although its organisation is already known 



in a general sense, a complete account of it may be interesting 



to naturalists, I submit the following memoir on the subject*. 



" A beautiful memoir on the structure of tlie Holothuria tubnlosa by 

 Tiedemanii was published in 1810. Edit. 



