'ON LIGHT. 26l 



(45.) The analysis into its prismatic elements of the 

 colour of any natural object, is readily performed by 

 examining through the refracting angle of a prism ot 

 perfectly colourless glass a rectilinear band or strip of 

 the colour to be analysed, so narrow as to have scarcely 

 any apparent breadth, and to appear as little more than 

 a coloured line. Placing this on a perfectly black 

 ground, parallel to the refracting edge of the prism, and 

 illuminating it as strongly as possible, it will be seen 

 dilated into a spectrum, or broad riband of colour, ex- 

 hibiting of course those coloured rays only which belong 

 to the composition of the tint examined. An exceed- 

 ingly convenient arrangement for this purpose is to 

 fasten across one end of a hollow square tube of metal 

 or pasteboard blackened within, of about an inch square 

 and twelve or fourteen inches long, a metal plate having 

 in it a very narrow slit parallel to one side, quite straight, 

 and very cleanly and sharply cut. At the other end 

 within the tube is to be fixed a small prism of highly 

 dispersive colourless flint glass, having its refracting 

 angle parallel to the slit, and so" placed that when the 

 tube is directed to the sky, or rather to a white cloud, 

 the slit shall be seen dilated into a clear and distinct 

 prismatic spectrum. In this of course all the prismatic 

 colours will be seen in their due order. But if, instead 

 of this, any coloured object as the leaf of a flower, for 

 instance, or a coloured paper, strongly illuminated by 

 direct sunshine (if necessary, concentred on it by a lens, 

 so, however, as not to scorch the object by the heat of 

 its focus), be placed so near to the slit as completely to 



