VISION. 



1437 



light travels a distance equal to eight times 

 the circumference of the earth, between two 

 beats of a clock. 



That white light was supposed by Newton 

 to be composed of seven colours is too well 

 known to need description here ; but the 

 proportion of each colour, according to the 

 observations of Newton and Fraiinhofer, is 

 as follows : 



Red - 



Orange 



Yellow 



Green 



Blue 



Indigo 



Violet 



Newton. 



- 45 



- 27 



- 40 



- 60 



- 60 



- 48 



- 80 



360 



Fraunhofer. 



- 56 



- 27 



- 27 



- 46 



- 48 



- 47 



- 109 



360 



It is to Sir David Brewster that we are in- 

 debted for our knowledge of the fact, that 

 the solar spectrum in reality consists of only 

 three primary colours, red, yellow, and blue; 

 each of which exists throughout the whole of 

 the spectrum ; and that the super-position of 

 these in different degrees of intensity in dif- 

 ferent parts produces the seven hues. The 

 proportion in which the primary colours com- 

 bine to form white light, is : 



Yellow, three parts. 



lied, five parts. 



Blue, eight parts. 



All the seven hues are possessed of different 

 degrees of refrangibility ; the red being least 

 refrangible, the violet the most. It appears 

 from Sir John Herschell's experiments that 

 just beyond the violet there exists a band 

 of coloured light of still greater refrangibility, 

 which he has denominated the lavender band. 

 The same great philosopher has also proved 

 that the coloured ravs in the spectrum differ 

 materially in the length and rapidity of their 

 undulations. 



From this table it will be seen that the sen- 

 sibility of the eye is confined within much 

 narrower limits than that of the ear, the ratio 

 of the extreme vibrations being nearly 1*58: 1, 

 therefore less than an octave, and equal to a 

 minor sixth. 



Sir William Herschell discovered that there 

 are rays in the solar spectrum which give rise 

 to the sensation of heat, independently of 



those of light, and these calorific rays are 

 most abundant a little beyond the red ex- 

 tremity of the spectrum, and gradually di- 

 minish towards the violet, beyond which they 

 are imperceptible ; an important practical fact 

 has recently, been discovered in relation to 

 these rays. It is well known that plants 

 growing in stove houses often suffer from the 

 scorching influence of the calorific rays, arid 

 when the great palm-house at Kew was about 

 to be erected, an elaborate series of experi- 

 ments was undertaken by Mr. R. Hunt, to 

 ascertain whether it would not be possible to 

 cut them off by means of a tinted glass. In 

 this he was fully successful, and discovered 

 that a glass tinted of a very pale yellow-green 

 colour by oxide of copper completely pre- 

 vented the permeation of all that class of 

 heat rays which exists below, and in the point 

 fixed at that of maximum calorific action ; as it 

 is to this class of rays that the scorching in- 

 fluence is due, the use of the glass described has 

 effectually protected the plants. The absence 

 of oxide of manganese commonly employed 

 in all sheet glass, is insisted on, it having been 

 found that such glass will, after exposure for 

 some time to intense sun-light, assume a 

 pinky hue, which is highly objectionable. 



To Dr. Wollaston we owe the discovery of 

 the existence beyond the extreme violet of 

 chemical rays, which are known to us solely by 

 their effects. It is to their action that the 

 fading of vegetable colours, and the blackening 

 of nitrate of silver, is due. The influence of 

 these chemical rays extends throughout the 

 spectrum, and to it the term actinism has been 

 applied. From the experiments of Mr. Hunt it 

 appears that actinism exercises an important 

 influence on the vegetable world, exciting ths 

 germination of seeds, and being essential to the 

 formation of the colouring matter of leaves. 

 The actinic principle is most energetic in spring, 

 when its stimulus is required to rouse dormant 

 vegetation from the repose of winter : as soon 

 as this is effected, the luminous rajs, with the 

 advance of the sun, become more active, and 

 the formation of woody fibre proceeds under 

 their particular agency ; but in autumn, the 

 actinic power having performed its part, is 

 no longer required, the whole energy of ve- 

 getation being concentrated under the in- 

 fluence of the calorific rays in the ripening 

 of fruits and perfecting of seeds. It has 

 been long known that the calorific and lumi- 

 nous rays were capable of extinction by means 

 of polarisation, but it has only been very re- 

 cently ascertained by Professor Wartmann 

 that the chemical rays are equally amenable 

 to polarisation under similar conditions. 



Those diversified colours which render the 

 floral world so attractive, which impart such 

 beauty to the feathered tribes and the legions 

 of butterflies, and in brilliant pigments re- 

 ward the labours of the chemist, are not pro- 

 perties inherent in matter itself, but arise 

 from the action of matter upon light, whereby 

 certain of the coloured rays which form white 

 light are reflected, whilst others are absorbed 

 or transmitted. Scarlet cloth, for instance 



