90 Prof. V. B. Lewes. [Feb. 1, 



and = d/r, where d is the diameter of the lens ; 

 hence = d/ 



The other columns of the table explain themselves. 



On the whole, I think it must be concluded that Insects do not see 

 well, at any rate as regards their power of defining distant objects, 

 and their behaviour certainly favours this view ; but they have an 

 advantage over simple-eyed animals in the fact that there is hardly 

 any practical limit to the nearness of the objects they can examine. 

 With the composite eye, indeed, the closer the object the better the 

 sight, for the greater will be the number of lenses employed to pro- 

 duce the impression; whereas in the simple eye the focal length 

 of the lens limits the distance at which a distinct view can be 

 obtained. 



The best of the eyes mentioned in the table would give a picture 

 about as good as if executed in rather coarse wool-work and viewed 

 at a distance of a foot ; and, although a distant landscape could only 

 be indifferently represented on such a coarse-grained structure, it 

 would do very well for things near enough to occupy a considerable 

 part of the field of view . 



II. " The Action of Heat upon Ethylene." By VIVIAN B. LEWES. 

 Professor of Chemistry in the Royal Naval College, Green- 

 wich. Communicated by Professor THORPE, F.R.S. 

 Received December 6, 1893. 



The decompositions of the simpler forms of hydrocarbons at an 

 elevated temperature have always been recognised as a question of the 

 greatest importance, as upon them is dependent a true conception of 

 many of the actions taking place in the manufacture of coal gas and 

 other kindred processes of destructive distillation. 



Ethylene has in most cases been chosen as the hydrocarbon which 

 would lend itself most readily to experimental researches upon tl 

 point, as, besides being one of the simplest, it is easily prepared, 

 is moreover found as one of the products in nearly all cases wht 

 organic compounds are subjected to distillation at high temperatui 



No sooner had the difference between ethylene and methane 

 recognised, than experiments were made by Deimann, Van Troostwyl 

 Lauwerenburg, and Bondt* to ascertain the action of heat upon 

 newly-formed compound, and the conclusions which they came 

 were that on heating no contraction in volume was observed, but tl 

 the tubes in which the decomposition was effected became coat 

 with a black deposit, and drops of an oily body were formed, the 



* ' Annales de Chimie et de Physique,' 1st series, vol. 21, p. 48. 



