lid Inquiries hi'o Coloured Light. 



In the firft place, it is known by Sir Ifaac Newton's ex- 

 periment, (Optic?;, book ii, part iii. prop. 7. p. 2^6.) " that 

 blacks are inclined to a blucifli colour may be feen from illu- 

 minating white pa]:)er by light refiefted from black fubftances, 

 for the paper will appear of a hlueijh luhlte." 



In confidering this obfervation on a faft as made by Sir Ifaac 

 Newton, the method to try the effeft of throwing a partial por- 

 tion of light on a (hade occurred to me. I darkened a room 

 partially, letting in a beam of light of five or fix inches breadth 

 Dy opening one of the fluitters. I then threw the (hade of 

 a ruler, bv the intercepted light of a candle, on a flieet of 

 paper: I then held this paper, with this (hade fo formed on 

 it, in the beam of light. The cfieft of this portion of day- 

 light thus thrown on the fhade of the ruler was, that the 

 Ihade exhibited a pale but decided blue. As I removed this 

 paper, with this fhade of the ruler thus thrown on it, further 

 back from the entrance of the light, but fVill in the fame beam, 

 this blue, without the alteration of anv one circumfiance but 

 a further deprivation of light, became of a ftronger, more 

 decided, deeper blue ; and To on gradually as I removed this 

 paper and fliade ftill further back, making a further depriva- 

 tion of light of a deeper and deeper but decided blue till it 

 went off into a black. This fliade, fuffering this gradual de- 

 privation of liiiht, exhibited all the gradations called blue 

 which are feen in the prifmatic fpectrum. Here, then, the 

 faft flands afcertnined, that blue is the effedl of a partial 

 li2:ht, and that a gradual deprivation of light gives all the 

 gradations of blue as they go off into dark. 



Mountains and hills, and even unelevated land, in the ho- 

 rizon exhibit, by the going off or deprivation of light, blue 

 tints in the moil dillant, though fuch are not the natural tints 

 of thcfeobjecls; and as thcfe objcds fuffer more and greater 

 deprivations of light in the evening, or from any other inci- 

 dental circumftance, thefe hues go through all the gradations 

 of blue. This any one may fee every day; and the painter, 

 if he is an artill, knows how to copy this phenomenon in 

 the arraneement of his colouring. 



Another circumftance in the landfcape of nature is ftill 

 farther to be obferved. As the fhade of evening comes on, 

 the blue in the dilfant horizon not only becomes deeper, 

 but, when the departing light fkirts the line of the horizon, 

 this blue, without any change in the flatc of the refractions, 

 or of the aftujil colour of the objects, becomes a flrong vivid 

 purple. The dark blue of llie clouds jufl above this fkirfing 

 of departing liirht, changes in like manner. The rays of the 

 reddifh-yeirow"'light aad^lhofe of this blue coming in coin- 

 cidence 



