have the power of producing chemical changes, such as are rendered familiar by the practice of Photography, are detected in their greatest activity. It has, therefore, been supposed that these fluorescent rays are the chemical rays rendered luminous by the alteration of their refrangibility. This view has received much support from the fact that the extra spectral rays are crossed with numerous dark lines, and that in the chemical impressions these lines are marked by unchanged spaces which exactly coincide with them. There is, however, much doubt of the correctness of this, since, in the uranium glass of such a thickness that these visible rays are quite absorbed, the chemical rays still pass. However, the whole question requires, and is receiving, the most searching investigation. The discovery of these phenomena, which are included under the term of Fluorescence, is of that interesting and important character, that it must be ranked as the most decided advance which has been made in physical optics since the days of Newton. It is not improbable that those rays of such high refrangibility may, although they are under ordinary circumstances invisible to the human eye, be adapted to produce the necessary degree of excitement upon which vision depends in the optic nerves of the night-roaming animals. The bat, the owl, and the cat, may see in the gloom of night by the aid of rays which are invisible to, or inactive on the eyes of man, or of those animals which require the light of day for perfect vision. It is a general law of the radiant forces, that whenever they fall upon any surface, a portion is thrown back or reflected at the same time as other portions are absorbed or transmitted. Upon this peculiarity appear to depend the phenomena of natural colour in bodies. The white light of the sun is well known to be composed of several coloured rays. Or rather, according to the theory of undulations, when the rate at which a ray vibrates is altered, a different sensation is produced upon the optic nerve. The analytical examination of this question shows, that to produce a red colour the ray of light must give 37,640 undulations in an inch, and 458,000000,000000 in a second. Yellow light requires 44,000 undulations in an inch, and 535,000000,000000 in a second; whilst the effect of blue results from 51,110 undulations within an inch, and 622,000000,000000 of waves in a second of time.[93] The determination of such points as these is among the highest refinements of science, and, when contrasted with the most sublime efforts of the imagination, they must appear immeasurably superior.