Indigo 658,000,000,000,000 " Violet 699,000,000,000,000 " Extreme violet 727,000,000,000,000 " Whatever theory we may adopt to explain the phenomena of light, we arrive at conclusions that strike the mind with astonishment and admiration. According to the corpuscular hypothesis, it was supposed that the molecules of light were endowed with the power of attraction and repulsion, that they possessed poles and centres of gravity like the earth, and that they had other physical properties that could only be given to ponderable matter. Starting with these notions, it is difficult to divest oneself of the idea of sensible size, or to induce the mind to conceive particles so extremely small as those of light would necessarily be if the theory of emission were accepted. If a particle of light weighed a grain, it would produce by means of its enormous velocity the effects of a cannon-ball weighing 120 lbs., travelling at the rate of 300 yards per second. How infinitely small would be these particles, seeing that the most delicate optical instruments are submitted to their action for years without being injured! If we are astonished at the extreme smallness and prodigious rapidity of the luminous molecules whose existence is necessitated by the corpuscular theory, the numerical results of the undulatory hypothesis are not less surprising. The extreme smallness of the distance between the waves, and the inconceivable quickness of their undulations, although both are easily calculated, must raise in the mind of the student feelings of the utmost wonder and admiration. Colour, then, simply results from the difference in the rate of vibration of the rays, as Professor Tyndall observes in his lectures on the “Analogy between Sight and Sound,” the impression of red being produced by waves that undulate a third less rapidly than those which produce the sensation of violet. ------------------------------------------------------------------------ CHAPTER IV. LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM. THE solar spectrum may be compared to a battle-field with an army drawn up upon it ready for action. In the centre we find the luminous rays, on one side the light troops which produce chemical effect, and on the other the heating rays, which may be compared to squadrons of heavy cavalry. Close by the light brigade are the magnetic rays, which are a corps of skirmishers, sometimes appearing, and at others hiding themselves from view in a very mysterious manner. But to drop metaphor, we shall find on examination of the spectrum that the three great forces—heat, light, and chemical effect—are regularly distributed over three different portions of this wonderful band of colour.