326 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q8. 



be agreed that these feeble analogies favour the persuasion, and that the mind will 

 not be satisfied until, by some new effort, flexion be re-united to the phenomena 

 of reflexion and refraction, by the unity of principle. 



Does our ignorance in the nature of the forces which produce these phenomena, 

 particularly in the nature of the repulsive force, and the want of agreement still 

 existing between the phenomena of flexion and the others, permit us to believe 

 in the physical cause long since intimated by Newton, lately renewed, and even 

 calculated, by Mr. B. ; I mean the difference in the size of the particles which 

 compose the different rays ? Are we in the right to countenance as an hypothesis, 

 Newton's theory of the fits of easy and difficult transmission ? This theory is only 

 a generalized expression of a well-observed fact. If the alternate transmissions 

 and reflexions depend only on the thickness of the transparent plates, either the 

 rays or the medium must alternate, and in equal tempuscules, in the opposite 

 dispositions. From experiments more varied it will appear if the thickness alone 

 influence it: it was with this view that the Abbe Mazeas undertook some experi- 

 ments, which have not yet given however any result, but which we might hope to 

 see followed with success, (Mem. des Sav. Etr., 1755). The theory of very thin 

 transparent plates it appears was conceived by Newton at 27 years of age, and he 

 did not publish it till 35 years later: for he alludes to it in his Lectures at Cam- 

 bridge in 1669, and the 1st edit, of his Optics was in the year 1704. As much as 

 it would be absurd to place even the most respectable authority on a parallel with 

 reason, so far is it just to require a very attentive examination of any one so much 

 reflected on. 



I shall conclude by observing, that the explanation I have proposed, according 

 to the Newtonian principles, of the phenomena observed by Mr. B., in the re- 

 flexion produced by a very small cylinder, does not injure the course he has taken 

 to explain the colours of natural bodies. His idea and that of Newton, in that 

 respect, are not in contradiction. It is not certain that the colours of natural 

 bodies are only produced in one manner; but, that they be produced, reflexion must 

 act on every particle of the bodies, in all sorts of angles. And I do not see that 

 Mr. B. has succeeded in trying, under many various angles, the reflexion he has 

 obtained by his small cylinders. It seems that he speaks only precisely of that 

 where the angle of incidence was about 77°> and consequently a very large one. 



XII. Of the Orifice in the Retina of the Human Eye, discovered by Prof. Soem- 

 mering. With Proofs of this appearance being extended to the Eyes of other 

 Animals. By Everard Home, Esq., F. R. S. p. 33'2. 



Having received a particular account of this discovery, in a very obliging letter 

 from Mr. Maunoir, an eminent surgeon at Geneva, which contains, I believe, the 

 material information published by Mr. Soemmering; I shall transcribe that part of 

 the letter, which is as follows: "The war being an obstacle to a free communication 

 between England and the continent, you are not perhaps acquainted with a new 



