OF THE LUMINOUS RAYS. 181 



plains over which are scattered the homes of honest 

 industry and domestic peace, intermingled with the 

 towers of those humble temples in which simple-hearted 

 piety delights to " bow the head and bend their knee " 

 these, all of these, may, by the sunbeam which illumi- 

 nates the whole, be faithfully pencilled upon our chemi- 

 cal preparations. 



Our ait enables us to do more even than this ; we 

 have but to present our sensitive tablet to the moon, 

 and she, by her own light, prints her mountains and her 

 valleys, and indicates with all truth the physical con- 

 ditions of her surface. 



Any reference to the chemical agency of LIGHT 

 the luminous rays as distinguished from the chemical 

 and calorific rays has been avoided. until we came to 

 the consideration of this particular question of chemical 

 change. 



Upon organic compounds, as, for instance, upon the 

 colouring matter of leaves and flowers, light does exert 

 a chemical power: and it is found that vegetable 

 colours are bleached, not by rays of their own colour, 

 but by those which are complementary to them. A red 

 dye fades under the influence of a green, ray, and a 

 yellow under that of a violet one, much more speedily 

 than when exposed to rays of any other colour ; and this, 

 it must be remembered, is due to the coloured ray 

 itself, and not to any actinic power masked, as it were, 

 behind the colour, as is generally believed.* It was 



* '* It may also be observed that the rays effective in destroying 

 a given tint are, in a great many cases, those whose union pro- 

 duces a colour complementary to the tint destroyed, or at least one 

 belonging to that class of colours to which such complementary 

 tint may be referred. For example, yellows tending towards 

 orange are destroyed with more energy by the blue rays ; blue by 

 the red, orange, and yellow rays; purples and pinks by yellow and 

 green rays." Sir J. F. W. Herschel, On the action of the rays of 

 the Solar Spectrum on Vegetable Colours : Phil. Trans., vol. cxxxiii. 

 1842. 



