J <0 Mr.Delaval on the Cauje of the 



I have thus briefly confidered the prefent ftate 

 of our knowledge, of the optical properties of 

 Colourlefs Tranfparent Subdances ; and the con- 

 dition and progrefs of the Arts which are fubor- 

 dinate to, and conne(fled with them. 



The induftry and attention of philofophers 

 has not hitherto, in any great degree, been ex- 

 crcifed on the confideration of Permanently Co- 

 loured Bodies. 



It will appear, in the courfe of this inquiry, 

 that the difpofition of fuch fubftances to exhibit 

 their colours, has not been examined by expe- 

 riments, which are the true and only means of 

 difcovering, and explaining, its caufe. And ic 

 -will be (hewn, that the hypothetical opinions, 

 which have been offered on this fubjeft, are not 

 fupported by aflual obfervations, and do not 

 account for, or agree with, the phenomena. 



From the imper^eftion of this branch of 

 Optics, and from the inftuence which its defefls 

 exert on thofe Arts, whofe improvement depends 

 upon a clear and juft conception of the nature, 

 preparation, and ufe of colouring materials, it 

 is not difficult to deduce the reafon of the 

 immaturity, and deficiency, under which they 



labour. 



So far, indeed, are we from having advanced 

 towards a ftate of perfection, that if we caft 

 our views back to the remotcft regions of Science, 

 we fhall find, from fuch a retrofpeft, that the 



rpoit 



