358 P, C. on the Colours tliut enter into the [Nov. 



If we direct a prism to a window, with the refracting 

 angle downward, the horizontal bars which separate the 

 panes of glass are seen fringed with colours; when the 

 prism is brought within a few inches of the window, the 

 colours above the bar are blue and violet, and below the bar 

 red and yellow ; the violet, though it appears upon the bar, 

 is seen, at this distance, separated from the red ; but upon 

 increasing the distance of the prism from the window, it 

 runs rapidly towards the red below the bar, and, by the 

 intersection of the two colours, forms a crimson. The 

 colours are now reduced to blue, crimson, and yellow, upon 

 the different bars of the window ; and between them the 

 white light appears to be admitted as usual. 



But the white light thus admitted has very different pro- 

 perties to ordinary light, and corresponds in every respect 

 with the white light seen upon the screen, between the 

 fringes of colours, in an early stage of the developement of 

 the spectrum, when it is formed by the direct light of the 

 sun in the usual manner. 



The shadows of all objects placed in this light, whether 

 before or behind the prism, are seen fringed with colours, 

 which increase in breadth with increase of distance, and 

 correspond exactly with the fringes seen upon the bars of 

 the window. This white light is refracted light : it pre- 

 serves its colour when uninterrupted, because the three 

 primitive coloui's intersect each other in the proportion 

 required to form white light ; but the rays of the three 

 colours have three different directions, or diverge at three 

 different angles, and the slightest interruption to the mutual 

 and successive compensation, which one image supplies to 

 the other, produces the appearance of colours. 



If an object be placed in white light formed of parallel 

 rays,* its shadow makes its progress to a distant screen, 

 bounded by lines parallel to these rays ; and when it falls 

 upon the screen it forms an undivided shadow, without any 

 appearance of colours ; but refracted white light, being 

 formed by the intersection of three surfaces of parallel 



• We are here speaking in general terms ; we know tliat it is impossible to 

 produce a surface of light formed of parallel rays ; every point of every object, 

 when illuminated with the direct light of the sun, receives rays from every part of 

 it : and the light reflected by the clouds must have still less pretension to this 

 character. 



