PROCEEDINGS OF SOCIETIES! 75 



in relief. The peculiar property of it is, that if the polished side be used to reflect 

 the rays of the sun on any plain surface, the reflection presents a perfect image of 

 the figures on the other side. This singular phenomenon has been explained in two 

 ways. Sir John Davis, the author of a work entitled "The Chinese: a General 

 Description of China and its Inhabitants," says that the effect is produced by etching 

 on the convex side a copy of the figures on the other ; that the surface is then highly 

 polished upon some soft, yielding substance, as cloth, so that the hollows may be 

 polished equally with the rest of the surface ; and that this etching is so light that 

 the figures will only be visible in very strong lights. 



Professor Faraday supposes that the mirror is cast in what is technically called a 

 " chiil," that, consequently, the density of the thick parts of the mirror is* less than 

 that of the thin portion ; that the surface in the process of polishing grinds away 

 unequally, as the denser parts will resist the grinding more than the other; that 

 thus hollows are produced on the convex surface corresponding in position to the 

 figures on the back, and that these form concave mirrors, which reflect the images 

 as described above. 



At Mr. Patterson's request, Mr. William Gray, a member of the society, under- 

 took to make a small casting to test, from actual experiment, the effect of a " chill." 

 His experiments, however, were not sufficiently extensive to enable him to arrive 

 at any satisfactory conclusion on that head ; but, from the difficulty he experienced 

 in getting the metals composing the alloy thoroughly mixed, he thought that the 

 unequal density on the metal, arising from imperfect mixture, would materially in- 

 terfere with like unequal density arising from the mode ef casting. Hence he in- 

 ferred that the less dense portions of the convex surface would not invariably cor- 

 respond in position to the figures on the back, and he suggested that, after the mirror 

 had been cast in a u chill," as Professor Faraday supposed, the upper surface was sub- 

 jected to some such process as that described by Sir J. Davis. By this means slight 

 hollows would be produced in proper positions ; and, afterwards, in the process of 

 grinding, the hollows would become greater when the density had varied according 

 to the law laid down by Faraday, and the inequalities of surface produced by the 

 acid in those parts of it where the density was regular would not be diminished by 

 grinding the surface on cloth. 



Mr. MacAdam read some notes on Physical Geography, relating to the distri- 

 bution of the great masses of land on the surface of the globe, and referred to some 

 theories which had been started in explanation. 



Mr. Patterson laid on the table a number of engravings of Lycian coins, and read 

 the following announcement respecting them : — " Sir Charles Fellows, being about 

 to publish some account of relative dates of the works of art discovered by him in 

 Lycia, is anxious to perfect, as far as possible, the portion illustrated by the ancient 

 coins of that province. Sir Charles sends impressions from the plates he is preparing, 

 and asks assistance in making additions or corrections, that he may be enabled to 

 to bring together, he believes for the first time, all the coins of Lycia before the age 

 of Alexander the Great." 



DUBLIN NATURAL HISTORY SOCIETY. 



FEBRUARY 10, 1854. 

 Dr. Gordon, M.R.I.A., in the chair. 



The usual routine business being disposed of, Mr. Kinahan presented to the 

 Society two volumes of Pennant's M British Zoology.'' 



Mr. Callwell, in proposing a vote of thanks for the donation, observed that works 

 of such a class were valuable to the Society as records of the state of zoology of that 

 day, and as comparison withh the present superiority of all works of the kind, and 

 of the advanced state of scientific research. The works of Pennant, however, are 

 well known for the accuracy of his details. 



