Comets and meteors have been submitted to the test of spectral analysis. The former erratic visitors have been but few and small since stellar spectrum analysis has been perfected. In January 1866, Mr. Huggins brought his apparatus to bear upon a small comet, which gave a somewhat unexpected result. When the object was viewed in the spectroscope, two spectra were distinguishable—a very faint continuous spectrum of the tail, showing that it reflected solar light, and a bright space towards the centre of the spectrum, indicating that the nucleous was self-luminous and gaseous. Mr. Alexander Herschel—the nephew and the grandson of Sir John and Sir William Herschel—has recently succeeded in obtaining indications of the composition of the meteors that people the heavens in the months of August and November. The principal result of his observations appears to be, that sodium in a state of luminous vapour is present in the trains left behind these singular bodies. Lightning has also been similarly examined, and lines showing that hydrogen and nitrogen were rendered luminous during the electrical discharge, were seen with great distinctness. In fact, the applications of the prism to scientific discovery are almost endless, and in describing them it is difficult to tell where to draw the line. Before quitting this subject, it will be as well to say a few words on the fluorescent rays of the spectrum, to which allusion has already been made towards the end of Chapter IV., Part II. It was there said that the chemical power of the spectrum extends to some distance beyond the extreme violet, a fact that may be readily proved by exposing a piece of photographic paper to the action of the dark portion of the spectrum. Professor Stokes found that there were means of rendering these rays visible to the eye by altering their rate of vibration. This he found was possible by passing them through the solutions of certain substances, such as sulphate of quinine, horse-chestnut bark, &c. We have already said, that light vibrating at the rate of from 458 to 727 billion times a second, was capable of exciting luminous sensations upon the optic nerve. The latter is the rate of vibration of the extreme violet ray, and it has been found that the eyes of many persons are not sufficiently sensitive to be influenced by it; it is, therefore, just probable that there are animals whose eyes are so much more sensitive than ours, that they can see rays that exist far beyond those seen by us. Now, as difference of colour is produced by difference in the rate of vibration, it follows that those whose eyes are sensitive enough to perceive the extreme violet rays, see tints of violet that are inappreciable by others.