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XXI IT. On the Decomposition of Li^^t into its most siiiiple 

 Elements, being Part of a JVork upon Colours. By 

 C. A. Prieur, hte a Colonel in the Corps of Engineers*. 



JL HE complex state of white light, which is tliat kind with 

 which we are most habitually acquain'Lcl, was discovered 

 by Newton. Since that period nothing important has been 

 added to the discoveries of that great philosopher; and the 

 objections made by some to this part of his experiments, 

 proceed either from want of examination, or From new ex- 

 periments ill understo'^d. They establish irresistibly the 

 various refrangibility of the particles of white light ; the dif- 

 ferent' colours of these particles ; and lastly, that the colours 

 of bodies are alone owing to the property they possess of 

 throwing out, reflecting, or transmitting some of these par- 

 ticles or rays more than others. 



Hence it follows, that, independently of the refractive 

 power, which is proper, in fact, for analysing light, every 

 coloured body presents by its action another method of 

 making this analysis more or less completely. 



Before discussing this last method, which is my present 

 object in particular, it may be useful to examine the first 

 method, with the view of thereby clearing up or confirm- 

 ing several peculiarities. 



Although the question has been often agitated, it has 

 never yet been satisfactorily resolved, viz. if there are really 

 seven distinct classes of colours in the solar spectrum, or 

 rather a single series of shades from the commencement 

 to the end, blended in an insensible manner. Some philo- 

 sophers are even uncertain if Newton haa distinctly affirmed 

 one or other of the propositions. 



On the first view, the opinion of this father of optics does 

 not appear doubtful j for he relates that a person, whose 

 sight was byetler than his own, was requested by himself to 

 draw across the spectrum lines on the confines of each co- 

 lour; and that the results of this operation, several times 

 repeated, agreeing tolerably well, he then fixed definitively 



• From /^tnales de Chimic^ torn. lix. p. 227. 



the 



