42 



VARIOUS MODES BY WHICH COLOUR IS PRODUCED. 



137. In the sunbeam different principles exist, some of which are visible to the eye 

 and others invisible. The conjoint action of the former communicates to us an im- 

 pression of what we denominate white light. But in this white light there are rays 

 endowed with the quality of exciting the sensations of colour, such as red, yellow, 

 green, blue. We are required to determine which of these principles is concerned in 

 the physiological change we are considering. It is necessary, therefore, to describe 



the SOLAR SPECTRUM. 



138. A beam of light coming into a room through an ordinary window is percepti- 

 ble from all parts, for dust or other heterogeneous particles which are always floating 

 in the air scatter the rays in all directions by reflexion, and enable them to operate on 

 the eye. This light constitutes what is designated by optical writers as white light. 

 To the ordinary acceptation of the term white, it does not. correspond ; it is the un- 

 changed, unaffected light of the sun. 



139. A beam of light coming through the painted window of a cathedral falls on 

 the ground, or on objects in its way, and communicates to them the various tints with 

 which the glass has been stained. The brilliant colours which are thus developed by 

 the action of the glass exist originally in the white light, and are made apparent by 

 the absorptive action of the medium through which they have come, in a way which will 

 be presently explained. 



140. There are also other modes by which, from white light, brilliant colours can be 

 produced. Thus, transparent media, such as gems, cut into certain shapes and polished, 

 when exposed to the light glisten with a play of colour. It is these brilliant hues which 

 give to the diamond its value as an article of female ornament. It is also in the same 

 way that the angular pieces of glass which are strung upon chandeliers and around gas 

 flames, emit in a brilliantly-lighted room so many fitful changes of colour. And in the 

 same way also Nature exhibits to us that most beautiful of all meteorological phenomena, 

 the rainbow, by refracting, reflecting, and dispersing the white light of the sun, and pro- 

 ducing a regular display of colours. These colours, arranged as in the rainbow, may 

 also be seen wherever a shower of drops of water is falling in a proper position as re- 

 spects the spectator and the sun ; they are often, therefore, visible when fountains are 

 playing; often in the sea-spray, when it is cast ashore by a brisk wind; often accom- 

 panying the bows of a steamboat which is moving rapidly through the water. 



141. There is a third mode by which, from white light, brilliant colours are produ- 

 ced. It is by the interference of rays; in the same manner that two sounds may be 

 so situated with respect to each other as to destroy one another's effect, and produce 

 silence ; or, as two waves upon water, when the concavity of the one corresponds with 

 the convexity of the other, destroy one another's effect; so may two rays or waves 

 of light be placed, in respect of each other, that, instead of re-enforcing each other's ef- 

 fect, they may produce darkness. This phenomenon, which in these different cases 

 passes under the general designation of interference, under ordinary circumstances, in 

 the case of light, is expressed by the production of brilliant colours. Such are the 

 beautiful and almost metallic tints that are seen on the wing-cases of certain coleopte- 

 rous insects, more especially certain beetles in the Southern States, which expose a 



