As the medium of vision—as the cause of colour—as a power influencing in a most striking manner all the forms of organisation around us, light presented to the inquiring minds of all ages a subject of the highest interest.
The ancient philosophers, although they lost themselves in the metaphysical subtleties of their schools, could not but discover in light an element of the utmost importance in natural operations. The alchemists regarded the luminous principle as a most subtile fluid, capable of interpenetrating and mingling with gross matter: gold being supposed to differ from the baser metals only in containing a larger quantity of this ethereal essence.[85] Modern science, after investigating most attentively a greater number of the phenomena of light, has endeavoured to assist the inquiry by the aid of hypotheses. Newton, in a theory, which exhibits the refined character of that great philosopher’s mind, supposes luminous particles to dart from the surfaces of bodies in all directions—that these infinitely minute particles are influenced by the attracting and repelling forces of matter, and thus turned back, or reflected, from their superficies in some cases, and absorbed into their interstitial spaces in others.
Huyghens, on the contrary, supposes light to be caused by the waves or vibrations of an infinitely elastic medium—Ether—diffused through all space, which waves are propagated in every direction from the luminous body. In the first theory, a luminous particle is supposed actually to come from the sun to the earth; in the other, the sun only occasions a disturbance of the ether, which extends with great rapidity, in the same manner as a wave spreads itself over the surface of a lake.
Nearly all the facts known in the time of Newton, and those discovered by him, were explained most satisfactorily by his hypothesis; but it was found they could be interpreted equally as the effects of undulation, with the exception of the production of colour by prismatic refraction. Although the labours of many gifted minds have been given, with the utmost devotion, to the support of the vibratory theory, this simple fact has never yet received any satisfactory explanation; and there are numerous discoveries connected with the molecular and chemical disturbances produced by the sun’s rays, which do not appear to be explained by the hypothesis of emission or of undulation.
In both theories a wave motion is admitted, and every fact renders it probable that this mode of progression applies not only to light, but to the so-called imponderable forces in general. Admitting, therefore, the undulatory movement of luminous rays, we shall not stop to consider those points of the discussion which have been so ably dealt with by Young, Laplace, Fresnel, Biot, Fraunhofer, Herschel, Brewster, and others, but proceed at once to consider the sources of light, and its more remarkable phenomena.[86]
The sun is the greatest permanently luminous body we are acquainted with, and that orb is continually pouring off light from its surface in all directions at the rate, through the resisting medium of space and of our own atmosphere, of 192,000 miles in a second of time. It has been calculated, however, that light would move through a vacuum with the speed of 192,500 miles in the same period. We, therefore, learn that a ray of light requires eight minutes and thirteen seconds to come from the sun to us. In travelling from the distant planet Uranus, nearly three hours are exhausted; and from the nearest of the fixed stars each ray of light requires more than six years to traverse the intervening space between it and the earth. Allow the mind to advance to the regions of nebulæ, and it will be found that hundreds of years must glide away during the passage of their radiations. Consequently, if one of those masses of matter, or even one of the remote fixed stars, was “blotted out of heaven” to-day, several generations of the finite inhabitants of this world would fade out of time before the obliteration could be known to man. Here the immensity of space assists us in our conception, limited though it be, of the for-ever of eternity.[87]
All the planets of our system shine with reflected light, and the moon, our satellite, also owes her silvery lustre to the sun’s radiations. The fixed stars are, in all probability, suns shining from the far distance of space, with their own self-emitted lights. By the photometric researches of Dr. Wollaston, we learn, however, that it would take 20,000 millions of such orbs as Sirius, the brightest of the fixed stars, to afford as much light as we derive from the sun. The same observer has proved that the brightest effulgence of the full moon is yet 801,072 times less than the luminous power of our solar centre.
The cultivators of modern science are a bold race; not contented with endeavouring to understand the physical earth, they are endeavouring to comprehend the condition of the solar surface. The mind of man can penetrate far into nature, and, as it were, feel out the mysteries of untraversed space. The astronomer learns of a peculiar condition of light, which is termed polarisation, and he learns by this, too, that he can determine if from a bright luminous disc the light is derived from a solid mass in a state of intense ignition, or from vapour in an incandescent condition. He adds a polarising apparatus to his telescopes, and he determines that the light we derive from the sun is due to an envelope of vapour—burning, in all probability—only with greater intensity, as the gas which we now employ. This Photosphere—as it has been called by the late French philosopher Arago, is found to be subjected to violent disturbances, and the dark spots seen on the sun’s disc are now known to be openings through this mysterious envelope of light, which enable us to look in upon the dark body of the sun itself.
Luminous phenomena may be produced by various means—chemical action is a source of light; and, under several circumstances in which the laws of affinity are strongly exerted, a very intense luminous effect is produced. Under this head all the phenomena of combustion are included. In the electric spark we have the development of light; and the arc which is formed between charcoal points at the poles of a powerful voltaic battery affords us the most intense artificial illumination with which we are acquainted. In addition to these, we have the peculiar phenomena of phosphorescence arising from chemical, calorific, electrical, actinic, and vital excitation, all of which must be particularly examined.