464 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Physical. 



By passing white light through a suitable prism, or through a 

 diffraction grating, it is decomposed into its constituents, which show as 

 a band of various coloured lights the well-known spectrum. By recom- 

 bining the coloured lights of the spectrum white light is again produced — 

 by recombining certain portions of it various coloured lights are produced. 

 But white light may be produced by combination of some only of the 

 spectral coloured lights, and although it will not be physically the same 

 as the original white light, it will produce the same effect on the eye. 

 Similarly, a coloured light of a particular hue may be produced by more 

 than one combination of spectral lights, and whilst physically they may 

 be different from each other, physiologically they will be similar. 



There is, however, another way of decomposing compound light, 

 whether white or coloured. If white light be allowed to impinge upon or 

 to enter certain substances, a portion of it may be selectively absorbed, 

 and the remainder reflected from the surface or transmitted through it. 

 The portion absorbed is lost as light, and the remainder, usually compound, 

 gives the substance its particular hue. If the substance absorbs all, or 

 nearly all, the light which falls upon it, it is black ; if it absorbs only a 

 portion of it, but all the spectral colours in the proportion in which they 

 exist in white light, the substance is grey or white. There is no sub- 

 stance which, if present in sufficiently large quantity, does not absorb all 

 the light which may enter it. Thus no light reaches the bottom of the 

 deep ocean. 



The colour of a substance when viewed by the light reflected from its 

 surface is not always the same as its colour when viewed by the light 

 which is transmitted through it. Such substances are said to be dichroic. 

 There are more than one dichroic pigment occurring in plants, but as they 

 do not exist there under such conditions that their dichroism is effective 

 it need not concern us here. 



There are three well-known characteristics of a colour : — 



1. The hue, often called the tone of a colour, whether a simple spectral 

 colour or a mixture of spectral colours, is determined by the wave-length 

 or the refrangibility of the coloured light or lights, in conjunction with 

 the quantities of each in its composition if the light be compound. Thus, 

 in the case of simple colours, all the colours of the spectrum differ from 

 each other in hue. In the case of a compound colour, such as lilac, it 

 may incline to the blue or the red side, according to the predominance of 

 red or blue in its composition. 



2. The luminosity or brightness of a colour is determined by the 

 actual amount of light of its particular hue which is given off from the 

 coloured surface, or which is transmitted through the coloured substance. 

 Thus, of two blues of the same hue, one may be brighter than the other. 

 In general, the same dye will cause more or less brightness in the colour 

 of a dyed fabric according as the dyeing solution used has been weaker or 

 stronger. And if there be two flowers both containing the same pigment 

 in different quantities, their colours will, in general, be of the same hue 

 but one will be brighter or more luminous than the other. 



3. The purity, fulness, saturation, or tint of a colour is determined by 



