OF THE PRODUCTION OF COLOURS. 157 



might tend greatly to eftablifh the Newtonian opinion, that 

 the colours of all natural bodies are fimilar in their origin to 

 thofe of thin plates ; an opinion which appears to do the 

 higheft honour to the fagacity of its author, and indeed to 

 form a very confiderable ftep in our advances towards an ac- 

 quaintance with the intimate conftitution and arrangement of 

 material fubftances. 



I have lately had an opportunity of confirming my former Difperfive 

 ©bfervations on the difperfive powers of the eye. I find that powersofthe 

 at the refpeclive diftances of 10 and 15 inches, the extreme 

 red and extreme violet rays are fimilarly refracted, the dif- 

 ference being exprefled by a focal length of 30 inches. Now 

 the interval between red and yellow is about one-fourth of 

 the whole fpe&rum; confequently, a focal length of 120 

 inches exprefles a power equivalent to the difperfion of the 

 red and yellow, and this differs but little from 132, which 

 was the refult of the obfervation already defcribed. I dp not 

 know that thefe experiments are more accurate than the 

 former one ; but I have repeated them feveral times under 

 different circumftances, and I have no doubt but the difper- 

 fion of coloured light in the human eye is nearly fuch as I 

 have ftated it. How it happens to be no greater, I cannot 

 at prefent undertake to explain. 



X. 



A Memoir on the Wax Tree of Louifiana and Pennfylvania. By 

 Charles Louis Cadet, of the College of Pharmacy at 

 Paris * . 



jt\ GREAT number of plants, fuch as the crotonfebiferum, Wax bearing 

 the tomex fcbifera of Loureiro, the poplar, the alder, the plants * 

 pine, and feveral labiated plants, afford a concrete inflamma- 

 ble matter by decoclion, more or lefs refembling tallow or 

 wax, that is to fay, a fixed oil faturated withoxigen. The 

 light matter, which is called the down of fruits, which filvers 

 the furface of prunes and other ftone fruits, is wax, as Mr. 

 frouft has (hewn. But the tree which prefents this fubftance 



* Tranflated fromthe Ann. de Chimie, XLIV. 140. 



in 



