A CHAPTER ON LIGHT AND COLOUR. 107 



years ago, manifested its presence in celestial bodies in a similar 

 manner, and its existence in the sun has long been known. 



13. — Bodies which are not self-luminous owe their visibility to 

 the Scattering of the light which falls on them, due to the 

 extreme irregularity of their surfaces. If they consist of material 

 so disposed as to reflect the incident light without considerable 

 selective absorption in the surface layers, as do snow, powdered 

 glass, etc., they appear of a more or less pure white. If the sur- 

 face layers are strongly absorptive, the scattered light is deficient 

 in particular colours, and the apparent colour of the body is 

 complementary to the colours absorbed. The light of the moon 

 and planets was believed for various reasons to be simply reflected 

 sunlight, and this conclusion is strongly confirmed by spectro- 

 scopic examination of these bodies. 



14-— Diffraction and Interference.— The light from a lumin- 

 ous point spreads uniformly in all directions in an isotropic medium, 

 and hence its brightness falls off in the same proportion as the 

 intensity of a sound, i.e., in the ratio in which the square of the 

 distance traversed increases. If we consider the form of the 

 wave-front at any time, it will be the surface of a sphere. At any 

 subsequent time the disturbance at any point of space may be 

 calculated, either by reference to the original source, or to the 

 wave-front referred to. Each point of this wave-front may be 

 considered as the origin of a disturbance, and, by adding the 

 effects of these disturbances, the total effect at any desired point 

 may be determined. It is found that the eflect due to any small 

 portion of the wave-front is evanescent, except in the neighbour- 

 hood of the direction perpendicular to the front of the wave, and 

 thus the existence of rays is accounted for. This practical anni- 

 hilation of one effect by another may best be studied by beginning 

 with a simple case. 



In all interference phenomena, the length of the wave of the 

 particular kind of light considered is most important. It is the 

 difference of wave-length of different colours which is the cause 

 of their different refraction, and consequently of the dispersion 

 produced by a prism, and the phenomena of interference are to a 

 large extent masked by the overlapping of the effects due to 

 differently coloured rays, resulting in a practical uniformity of 



