390 Mr. T. Zeno on the Changes in the 



conylene was obtained. It is a yellowish oil, of specific gravity 

 1*57, with an odour like mustard. Its formula is £ 8 H 14 Br 2 . 

 When this substance, bromide of conylene, is treated with hydrate 

 of potash, it is decomposed into bromide of potassium and a 

 colourless oily substance which is specifically lighter than water, 

 and which is the oxide of conylene, G s H 14 0. 

 The author is still engaged in the investigation of this substance. 



LI I. On the Changes in the Apparent Size of the Moon. 



By T. Zeno. 



To Professor Tyndall. 

 Sir, Guy's Hospital, October 8th, 1862. 



MY attention having been called to a letter in the May 

 Number of the Philosophical Magazine, "On the Changes 

 in the Apparent Size of the Moon," where reference is made to 

 my explanation of the phenomenon (Recreative Science, No. 24), 

 I find it necessary to ask your indulgence while I make some 

 allusion to that reference, as well as to the novel and ingenious 

 experiment of your correspondent Mr. Lewis. 



First, with regard to the reference to my explanation, it is 

 obvious that, to satisfy the "popular mind" as to the subjective 

 character of the phenomenon, it would have been better to have 

 quoted my second experiment, that of looking at the rising sun 

 till an ocular spectrum was obtained, and then directing the eye 

 to the zenith, where this spectrum immediately diminished, to 

 enlarge again on looking to the opposite horizon. Secondly, 

 with regard to the experiment of Mr. Lewis, it must be noticed 

 that, while it necessarily involves the condition of binocular vision, 

 the phenomenon it proposes to illustrate is equally exhibited to 

 monocular vision ; the experiment nevertheless, so far as I can 

 judge, very well illustrates that law by which, when two bodies 

 subtend equal angles, both bodies being of the same size, but 

 one assumed to be nearer than the other, the (assumed) nearer 

 appears to be smaller. And though, consequently, the moon in the 

 zenith would appear smaller if the senses assumed it to be nearer, the 

 experiment of your correspondent does not hint at the latter 

 condition. Now as the gravamen of the evidence lies here, you 

 will perhaps allow me to state briefly why it is that the luminary 

 (seen with one eye or with both eyes) in the zenith is felt to be 

 nearer than it is when seen in the horizon. I say felt, not 

 'reasoned, inasmuch as this is a sensory perception which no 

 intellection can alter, seeing the setting or rising sun is no less 

 enlarged to the eye of an astronomer (whose reason rejects the 

 illusion) than it is to the eye of the most uninformed rustic. 



I propose that the luminary appears nearer at zenith than 

 horizon, because, first (A), the visible sky appears nearer at zenith 



