Sir D. Brewster on a Rhombohcdral Crystallization of Ice. 245 



tensity, it follows that different parts of the retina have dif- 

 ferent degrees of sensibility to light. The lines which form 

 the network are probably thicker than the interstices between 

 them, and consequently less susceptible to luminous impres- 

 sions. In like manner the interstices nearest to the foramen 

 cenlrale are probably thinner than those more remote, and 

 hence it is easy to understand why they exhibit a greater sen- 

 sibility and a more rapid change of colour. If these views are 

 correct, we not only obtain a general explanation of the phe- 

 nomena which we have described, but of many others which 

 have hitherto perplexed the optical physiologist, and among 

 these we may enumerate the phenomena of oblique vision, 

 and the superior distinctness of objects when they are seen di- 

 rectly along the axis of the eye. 



In a former paper I had occasion to mention a very re- 

 markable fact, which I had long ago discovered, that the in- 

 tensity of a given light may be increased physiologically by 

 causing it to act upon the retina by successive impulses of a 

 given duration. Those who may repeat the preceding ex- 

 periments will obtain an ocular demonstration of the truth of 

 this new property of light. The maximum physiological iri— 



T 



tensity seems to take place when — is nearly equal to D + d. 



Belleville, Feb. 20, 1834. 



XL 1 1. Account of a Rhombohcdi al Crystallization of Ice. 

 By Sir David Brewster, LL.D., F.R.S., V.P.R.S. Ed. 



"\TARIOUS crystallizations of ice have been observed and 

 * recorded ; but the primitive form of its crystal has not 

 yet been determined. I have long ago shown that ice possesses 

 double refraction; that it has only one axis ; and that this axis 

 is perpendicular to the surface of well crystallized plates of ice 

 formed on the surface of still water. Hence it follows that the 

 primitive form of this kind of ice must be either a rhombohe- 

 droii, a right prism with a square base, or a pyramid with a 

 square base. Crystals of ice of all these forms have been seen, 

 I believe, by different individuals either in hoar frost or in icy 

 caverns; but in no case has the optical structure of these cry- 

 stals been submitted to examination. 



On the forenoon of fche 2nd of February, after a night of 

 very slight frost, I observed a circular basin of water, which 

 was protected from the agitation of the air by a stone parapet, 

 covered with a sheet of perfectly transparent ice without a 

 single air bubble. Upon examining it more narrowly 1 was 



