The velocity of the luminous undulations deduced by Romer from the eclipses of the satellites of Jupiter, is proved to be about 192,500 miles in a second: in other words, 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 :— 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.
Red, 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 rays 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 P.58:1, therefore less than an octave, and equal to a minor sixth.
Sir William ITerschell 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, and 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 fluenceis due, the use of theglass 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 au important influence on the vegetable world, exciting the 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 rays, 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.