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. From whatever source we procure light, it is the same in character, differing only in intensity. In its action upon matter, we have the phenomena of transmission, of reflection, of refraction, of colour, of polarisation, and of vision, to engage our attention.