216 



THE POPULAR EDUCATOR. 



Pig. I 1 



is about 91,000,000 miles, how it is that light can achieve this 

 enormous interspace in eight and a-half minutes, when a rail- 

 way train, going at a speed of thirty miles per hour and starting 

 on the 1st of July, 1869, would not reach the sun until the end 

 of the year 2207. 



The velocity of light is, according to Mons. Foucault, 185,177 

 miles per second ; but his experi- 

 ments were tried through distances 

 on the earth. Homer, by astrono- 

 mical observations, gives a greater 

 rate of speed, viz., 192,500 miles per 

 second ; and this is very nearly the 

 same as that determined subsequently 

 by Bradley, who calculated the velocity 

 to be 191,515 miles per second. 



The immortal Newton explained 

 the manner in which light travelled 

 through space by supposing, firstly, 

 that it consisted of material particles 

 or corpuscles, so rare and subtile 

 that no balance, however exquisitely 

 devised, could be made to appreciate 

 them ; and, secondly, that these cor- 

 puscles were shot out from the sun 

 and all luminous bodies with amaz- 

 ing velocity, and in consequence of 

 their rarity passed bodily through 

 solids, liquids, or gases, and caused 

 the sensation of vision by ultimately 

 finding their way to and impinging on 

 the expanded nerve or retina of the 

 eye. This theory is called the cor- 

 puscular or emissive theory of light, 



and when applied even to explain the reflection or refraction 

 of light it fails to do so, and with the more complicated 

 phenomena such as the colours of thin plates, and especially 

 with polarised light the difficulty of reconciling the theory 

 with the facts increases. Thus it has come to pass that the 

 hypothesis of Newton is now rejected and that another one 

 is deservedly substituted for it. 



It was probably in consequence of 

 the great musical knowledge of 

 Huygens that this celebrated astro- 

 nomer was led to oppose the New- 

 tonian theory, and to insist that the 

 mechanism of sound and light was 

 identical, and that it was in both 

 oases an undulating or wave motion. 

 Euler, like Huygens, was perfectly 

 acquainted with the philosophy of 

 Bound, and he also, being a great 

 mathematician, opposed the emissive 

 theory. The arguments of these 

 great men, however weighty, would 

 have had but little effect if they had 

 not been succeeded by the practical 

 experiments and mathematical powers 

 of Dr. Thomas Young, who reasserted 

 and revived the theory of undulations 

 about the beginning of the present 

 century. The wave theory of light 

 starts with the assumption that there 

 ia an infinitely rare medium filling 

 space, and contained in all solid, fluid, 

 and gaseous bodies, which is called 

 ether. The latter is not light, air is 

 not sound, water is not necessarily 

 a wave; but light is produced in the 



ether by the setting up of an undulating or wave motion by 

 the vibration or trembling of the particles of the luminous 

 body. The sun, or rather the molecules of which its photo- 

 sphere or source of light is composed, is supposed to be in a 

 vibratory condition ; these vibrations are communicated to the 

 ether, and transmitted in the form of waves, which may 

 ultimately dash or gently impinge against the_ retina. If they 

 strike violently as. for instance, when the astronomer forgets 

 to use the coloured glass or tank of diluted ink whilst viewing 

 the sun through his telescope then great damage, even blind- 



B 



Fig. 2. 



Fig. 3. 



ness, may be caused. If the waves move softly, as in diffused 

 daylight, the gentle ripple of the ether dies away harmlessly on 

 the nerve of vision. 



The ether is not only the incarnation of levity, but of elas- 

 ticity, and just as sound travels more quickly through an elastic 

 substance, so the waves of light are propagated from the sun 

 at the enormous velocity already 

 stated. 



A rod of wood is infinitely lesa 

 dense and lighter than a bar of cast- 

 iron, but in consequence of its elas- 

 ticity being greater than that of the 

 metal, the vibrations of sound travel 

 more quickly through its particles 

 than they do through the cast-iron. 

 The luminiferous ether is supposed 

 to possess a spring or elasticity 

 greater than that of any other created 

 matter, and thus the first question, 

 "Why does light travel so fast?" 

 is answered by the wave theory. 

 Some acoustic experiments may assist 

 the learner to appreciate the idea of 

 an undulating medium. 



If the wetted finger is drawn around 

 the edge of an ordinary finger-glass 

 a loud sound is emitted, and the vi- 

 bration of the glass is easily shown 

 by suspending a bit of cork with a 

 thread, and allowing it to touch the 

 trembling glass. As long as the 

 sound lasts, the cork will be driven 

 away by the constant succession of 



blows or impulses it receives. (Fig. 1.) Substitute ideally the 

 sun or any other self-luminous body for the trembling glass, 

 and the analogy between the two is at once established, only 

 instead of air being set in motion it is the hypothetical ether 

 which is affected by the vibrating molecules of the luminous 

 body. 



Ether conveys the vibrations like a solid substance ; such a 

 conveyance or travelling of vibrations 

 may be shown by placing the ear at 

 one end of a long piece of timber, 

 and scratching with a nail at the 

 other, or by placing a watch at the 

 extremity of a light wooden rod and 

 listening at the other. The vibrations 

 set tip in a finger-glass may soon be 

 communicated to another, by connect- 

 ing the two together with a wooden 

 rod, which may be of any convenient 

 length. In this experiment the lumi- 

 nous body is represented by the glass 

 A, the ether by the rod B, and the body 

 upon which the luminiferous ether 

 impinges by the glass c. (Fig. 2.) 



A musical note is produced when 

 the impulses in the air are suffi- 

 ciently frequent, and its pitch de- 

 pends upon the number of aerial 

 waves which recur in a second. If 

 the undulations follow each other very 

 rapidly, an acute sound is heard ; or 

 the reverse, a grave one, when the 

 recurrences of the waves are less 

 rapid. Tyndall says, "Colour is to 

 light what pitch is to sound; the 

 colour of light depends on the number 



of ethereal waves which strike the eye in a second. Thus the 

 sensation of red is produced by imparting to the optic nerve 

 four hundred and seventy-four millions of millions of impulses 

 per second, while the sensation of violet is produced by impart- 

 ing to the nerve six hundred and ninety-nine millions of million? 

 of impulses per second. In the ' emissive theory ' numbers not 

 less immense occur." "Now," remarks Herschel, "is there 

 any mode of conceiving the subject which does not call upon us 

 to admit the exertion of mechanical forces which may well be 

 called infinite ? " 



