i873«] Colours and their Relations. 85 



substance or object. The eye which is usually impressed 

 simultaneously by luminous waves of every degree of length, 

 causing the perception of mere brightness, is in this case 

 separately impressed by waves of different definite lengths, 

 the vibrations of which are so adjusted to those of which the 

 optic nerve is capable, as to excite in us the perception of 

 definite colour. 



The laws which regulate the dispersion of light in passing 

 through diverse media are exceedingly curious ; and some 

 of the more important of them have been noticed in a 

 previous essay on the spectroscope in the " Quarterly Journal 

 of Science " for January, 1872. 



A familiar example of the production of adventitious 

 colours by the separation of wave from wave of the ether, 

 where the object which affects the separation is itself colour- 

 less, is exhibited in the rainbow. This phenomenon is pro- 

 duced by the action of falling rain-drops, or of the spray 

 from waterfalls on the sunbeams. The ethereal waves, on 

 entering a rain-drop, become separated one from another, 

 owing to their unequal refrangibility, and their passing 

 through different thicknesses of the watery medium. Being 

 reflected from the posterior surface of the drop in this sepa- 

 rate condition, they undergo further separation in passing a 

 second time through the water; so that, on emergence, the 

 differently coloured waves reach the eye separately. The 

 mode of formation of the primary and secondary bows will 

 be found explained on mathematical principles in the 

 " Edinburgh Encyclopaedia," vol. xv., p. 616. 



Haloes round the sun and moon are also examples of the 

 same sort of adventitious colours, and their explanation 

 depends on similar principles; only the objects by which 

 the separation of the ethereal waves is effected are thought 

 to be not rain drops, but minute frozen particles of water. 

 It is supposed that a stream of air, charged with moisture, 

 at a low temperature, comes into contact with a denser, 

 drier, and colder stratum, by which the particles of moisture 

 become suddenly frozen into very minute crystals, which are 

 sustained floating in the atmosphere at a considerable height, 

 in a thin semi-transparent layer, forming a sort of veil be- 

 tween the observer and the sun or moon. The explanation 

 of these -phenomena on mathematical principles will be 

 found in the " Edinburgh Encyclopaedia," vol. x., pp. 616, 617. 



To similar causes are to be attributed the rarer and more 

 striking phenomena of the parhelion or mock-sun, and the 

 paraselene or mock-moon. The author, many years ago, 

 once enjoyed an opportunity of seeing a parhelion of great 



