"Royal Society of Lofitl: >7. 2&$ 



been able to compare the difperfive powers of feyera-l fuch 

 fubfianccH with that of plate glafs. He has alio arranged a 

 number of fubitances in a table in the order of their difperfive 

 powers, "at. a given deviation; an order materially diilerent 

 from that of their refractive deniity. A very important ob* 

 fervalion concludes this part of the ellay. Dr. YVoilaiton 

 obferves, that, by looking through a priim at a ciifiant cre- 

 vice in a window-mutter, the division of the {plectrum may 

 be.ieen more diftinetly than by any other, method;, and that 

 the colours are then only four, red, yellowilh green, blue, 

 and violet, in the linear proportions of the numbers 16, 23, 

 36, 25 ; and that thefe proportions will be the lame whatever 

 retractive fublta nee be employed, provided that the inclina- 

 tion of the priim remain unchanged. In the light of the 

 lower part of a candle, the fpectrum is diftinguiflied by dark 

 fpaces into five dihinct portions. 



Another communication from Dr. Wollafton, on the 

 oblique refraction of Iceland crystal, was alio read the fame 

 evening. 



• It contains a confirmation of the experiments of Huvgens 

 011 this fubftance, with additional evidence, deduced from 

 the fuperiority of Dr. Wollafton's mode of examining the 

 powers of refraction. He obferves, that Dr. Young has ah 

 ready applied the Buygenian theory with considerable fuc- 

 cefs to the explanation of feveral other optical phsenomena-, 

 and that it appears to be ftrongly fupported by fuch a coinci- 

 dence of the calculations deduced from it, with the reiults 

 of thefe experiments, as could icarcely have happened to a 

 falfe theory. 



On the 1 ft of July was read a paper, by Dr. Young, F.K.S^ 

 giving an account of fome cafes of the production of colours 

 not hitherto defcri bed. 



A paper -on the composition of emery, by Smithfou Ten- 

 nan t, Efq.' F.R. S. was alfo read. 



Mr. Tennant -finds that, emery is difTolved with fome diffi- 

 culty in a ftrong heat by carbonate, of foda, and, after the 

 fubfidence of a little iron, the. earth contained in the folution 

 is almost purely argillaceous. This rcfult is exactly similar 

 to Mr. Klaproth's analysis of diamond fpar or corundum. 

 From 100 parts Mr. Tennant procured .80 of argil,- 3 of (ilex, 

 and 4 of iron, with an uhdiiTolved refiduum of 3 parts, and 

 a lots of io, great care having lx?en taken to feparate the 

 parts attracted by the magnet : fome portions however con- 

 tained almolt one third of iron. . The hardnefs of emery and 

 diamond fpar appears to be equal. The emery ufed in Eng- 

 land ifr brought principally from the iftand of JNfax©*; it is 



imported 



