14 REPORT — 1842. 



artificially polished. I believe (though I have made very few calculations relative 

 to the point), that the former class of experiments will be perfectly explained by the 

 theory; the latter I am certain cannot, nor ought ice to expect that they should; for 

 the process of artificially polishing must necessarily occasion small irregularities by 

 exposing little elementary rhombs with their faces inclined to the general surface, 

 and the action of these faces may produce the unsymmetrical effects which Sir David 

 Brewster notices as so extraordinary. If this does not account for such effects, I do 

 not know what will." Had Prof. M'Cullagh communicated to me this explanation of 

 the incapacity of the undulatory theory to account for the extraordinary unsymme- 

 trical phenomena which I described to the British Association, and which exist to a 

 much greater extent than I described ; or had it been contained in the two abstracts 

 of his memoir, with which I was familiar, I could at once have removed the difficulty 

 referred to in the preceding note. The view he has taken of the action of an arti- 

 ficially polished surface of Iceland spar, is a mistaken one. The exposure of ele- 

 mentary rhombs with faces oblique to the general surface, would show themselves in 

 separate rays inclined to the principal pencil, especially in solar light. It could not for 

 an instant be overlooked by an experienced observer. Such faces I can produce at 

 pleasure, by a slight chemical action upon the surface, whether polished by crystal- 

 lization or by art ; and it is impossible to confound the pencil which they reflect, with 

 that which is given by the general surface. It is useless, however, to pursue this ar- 

 gument any further, because I have obtained exactly the same results in using natural 

 faces, and in using artificial ones, and especially on planes perpendicular to the axis 

 of the crystal, where I have found the same results with the natural faces of the Chaux 

 carbonatee basee of Hauy, and with those produced by artificial grinding. In this case 

 the coincidence is still more remarkable, as the very friction of the finger is capable 

 of developing on this surface the faces of elementary rhombs ; but the reflexions from 

 these never disturb in the slightest degree the physical action of the general surface. 

 I have no doubt that Prof. M'Cullagh will concur in the accuracy of these views, and, 

 with that candour which distinguishes him, will acknowledge, as he has almost done 

 already in the preceding note, that the undulatory theory is, generally speaking, in- 

 capable of explaining the phaenomena of crystalline reflexion. 



On a very curious fact connected with Photography, discovered by M. Moser 

 of Kbnigsberg, communicated by Prof. Bessel to Sir D. Brewster. 

 Sir D. Brewster said, he was requested to communicate an account of some remark- 

 able facts connected with the theory of photography. A new process of producing 

 photographic impressions had been discovered by Dr. Mbser of Kbnigsberg ; and an 

 account of the discovery had been brought to this country by Prof. Bessel, who re- 

 ceived it from the discoverer himself. The subject was most important, and it would 

 have been a great misfortune if the Physical Section had separated without being 

 made acquainted with it. The following were the general facts connected with it :— 

 A black plate of horn, or agate, is placed below a polished surface of silver, at the 

 distance of one-twentieth of an inch, and remains there for ten minutes. The surface 

 of the silver receives an impression of the figure, writing, or crest, which may be cut 

 upon the agate or horn. The figures &c. do not appear on the silver at the expiration 

 of the ten minutes, but are rendered visible by exposing the silver plate to vapour, 

 either of amber, water, mercury, or any other fluid. He (Sir D. Brewster) had heard 

 Prof. Bessel say, that the vapours of different fluids were analogous to the different 

 coloured rays "of the spectrum ; that the different fluids had different effects, cor- 

 responding to those of the spectrum ; and that they could, in consequence of such cor- 

 respondence, produce a red, blue, or violet colour. The image of the camera obscura 

 might be projected on any surface, — glass, silver, or the smooth leather cover of a 

 book, without any previous preparation ; and the effects would be the same as those 

 produced on a silver plate covered with iodine. 



On the Dichroism of a Solution of Stramonium in 2Ether. 

 By Sir David Brewster. 

 The solution was yellow by transmitted light, but green by reflected light. 



