10 XlEPOllT 186J;. 



On the Connexion between the Form and Optical Properties of Crystals. 



Bij A. Catton. 

 It was the object of this paper to give an account of the results of investigations 

 which have had for their object the discovery of the connexion between the form 

 and optical properties of ciystals. It is believed that, in the results here given, 

 some of the principal difficulties of this important proljlem have been overcome. 

 The first step towards the solution of this problem was made by Su' David Brewster 

 in 1818. He discovered that crystals belonging to the prismatic, oblique, and 

 anorthic systems are biaxal ; those belonging to the pyramidal and rhombohedral 

 systems imiaxal ; while crystals of the cubic system do not possess double refrac- 

 tion (a fact which had been previously stated by Hauy). Li this paper is con- 

 sidered iu detail the connexion between the form and optical properties of crystals 

 belonging to the prismatic system. The investigation of the optical properties of 

 ciystals belonging to the oblique system is still in progress. The following is the 

 method employed in this investigation: — Each crystal is referred to three rect- 

 angular axes, one axis being perpendicidar to the plane of the optic axes, the other 

 two being the internal and external bisectors of the angle between the optic axes. 

 The new parameters are calculated by means of formulae investigated in a paper on 

 the " Rhombohedral system ; " and thence the angle between the optic axes is 

 found, as if the crjstal belonged to the prismatic sj^stem, by means of the formula) 

 given in this paper. The angle between the optic axes of one mineral belonging 

 to the oblique system has been calculated by this process ; and the calcidated has 

 been found to agTee approximately with the observed angle. If this should prove 

 to be o'enerally the case, it will not only be a solution of the problem which lorms 

 the suljject of this note for crystals of the oblique system, but it mil prove that 

 these crystals are formed according to the same laws of symmetry as crystals of the 

 prismatic system. 



Photo-SmJpture. By A. Claudet, F.B.S. 



After having explained the advantages of photography and its progress, tho 

 author described in what manner it has been applied to sculpture. 



This beautiful application of photography is called Photo- Sculjiture, and is the 

 invention of M. WiUeme, an eminent French scidptor. 



The story of tho invention may be told in a few words. M. Willome was in the 

 habit, whenever he could procure photographs of his sitters, of endeavouring to 

 commimicate to the model the correctness of those unerring types. But how should 

 he raise the outlines of flat pictm-os into a solid form ? 



Yet these single photographs, such as they were, could serve hira to measure 

 exactly profile outlines. He could, indeed, by means of one of the points of a 

 pantograph, follow the outline of tho photograph, while, with the other point 

 directed on the model, he ascertained and corrected any error which had been 

 communicated to his work diuing the modelling. What he could do with one 

 view, or one single photograph of the sitter, he might do also with several other 

 views, if he had them. This was sufficient to open the inquiry oi an ingenious 

 mind. He saw at once that if he had photographs of many other profiles of the 

 sitter, taken at the same moment, by a number of cameras-obscuras placed around, 

 he might alternately and consecutively correct his model by comparing the pro- 

 file outline of each photograph with the corresponding- outline of the model. Such 

 was the origin of this discovery. But it soon naturally occuiTed to him that, in- 

 stead of correcting his model when nearly completed, he had better work at once 

 with the pantograph upon the rough block of clay, and cut it out gradually all round 

 in following one after the other the outline of each of the photographs. 



Now supposing that he had twenty-four photographs, representing the sitter in as 

 many points of view (all taken at once), he had but to turn the bloclc of clay, after 

 eveiT operation, -^V^b of the base upon which it is fixed, and to cut out the next 

 profile, imtil the block had completed its entire revolution, and then the clay was 

 transformed into a perfect solid figure of the twenty-four photographs — the statue 

 or bust was made. 



As an illustration of the process, the author has executed a bust of the President 

 of the Association, Sir Charles Lyell, which was exhibited to the Section. 



