SIX LECTURES ON LIGHT. 



ireans cf the syren the number of vibrations 

 due to any musical no'e, whether it be that of 

 an instrument, of the human voice, or of a 

 flying insect, may be accurately determined. 



In front of our lamp now stands a very 

 homely instrument, S, Fi<?-. 4, of this charac- 

 ter. The perforated disk is turned by a 

 wheel and band, and, when the two sets of 

 perforations coincide, a series of spots of 

 light, sharply denned by the lens L, ranged 

 on the circumference of a circle, is seen upon 

 the screen. On slowly turning the disk, a 

 flicker is produced by the alternate stoppage 

 and transmission of the light. At the same 

 time air is urged into the syren, and you hear 

 a fluttering sound corresponding to the flick- 

 ering light. But, by augmenting the rapid- 

 ity of rotation, the light, though intercepted 

 as before, appears perfectly steady, through 

 the persistence of impressions upon the 

 retina ; and, about the time when the optical 

 impression becomes continuous, the auditory 

 impression becomes equally so ; the puffs 

 from the syren linking themselves then to- 

 gether to a continuous musical note, which 

 rises in pitch with the rapidity of the rota- 

 lion. A movement of the head causes the 

 image of the spots to sweep over the retina, 

 producing beaded lines : the same effect is 

 produced upon our screen by the sweep of a 

 looking glass which has received the thin 

 beams from the syren. 



In the undulatory theory, what pitch is to 

 the ear, cclor is to the eye. Though never 

 seen, the lengths of the waves of light have 

 been determined. Their existence is proved 

 .by their effects, and from their effects also 

 their lengths may be accurately deduced. 

 This may, moreover, be done in many ways, 

 ami, when the different determinations are 



compared, the strictest harmony is found to 

 exist between them. The shortest waves- of 

 the visible spectrum are those of the extreme 

 violet ; the longest, those of the extreme 

 red ; while the other colors are of intermedi- 

 ate pitch or wave-length. The length of a 

 wave of the extreme red is such that it would 

 require 36,918 of them placed end to end to 

 cover one inch, while 64,631 of the extreme 

 violet waves would be required to span the 

 same distance. 



Now, the velocity of light, in round num- 

 bers, is 190,000 miles per second. Reducing 

 this to inches, and multiplying the number 

 ihus found by 36,918, we obtain the number 

 of waves of the extreme red in 190,000 miles. 

 All these "waves enter the eye, and hit the 

 retina fit the back of the eye in one second. 

 The number of shocks per second necessary 

 to the production of the impression of red is, 

 therefore, four hundred and fifty-one millions 

 of millions. In a similar manner, it may be 

 found that the number of shocks correspond- 

 ing to the impression of violet is seven hun- 

 dred and eighty-nine millions of millions. 

 All space is rilled with matter oscillating at 

 such rates. From every star waves of these 

 dimensions move with the velocity of light 

 like spherical shells outwards. And in the 

 ether, just as in the water, the motion of 

 cveiy particle ij the algebraic sum cf all the 

 separate motions imparted to it. Still, one 

 motion does not blot the other out ; or, if 

 extinction occur at one point, it is atoned for 

 at some other point. Every star declares by 

 its light its undamaged individuality, as if it 

 alone had sent its thrills through space. 



The principle of interference applies to the 

 waves of light as it does to the waves of 

 water and the waves of sound. And the 

 condi ions of interference are the same in all 

 three. If two series of light-waves of the 

 same length start at the same moment from 

 a common origin, crest coincides with crest, 

 sinus with sinus, and the two systems blend 

 together to a single system of double ampli- 

 tude. If both series start at the same mo- 

 ment, one of them being, at starting, a whole 

 wave-length in advance of the other, they 

 also add themselves together, and we have 

 an augmented luminous effect. Just as in 

 the case of sound, the same occurs when the 

 one system of waves is any even number of 

 semi-undulations in advance of the other. 

 But if the one system be half a wave-length, 

 or any odd number of half wave-lengths in 

 advance, then the crests of the one fail upon 

 the sinuses of the other ; the one system, in 

 fact, tends to lift the particles of ether at the 

 precise places where the other tends to depress 

 them ; hence, through their joint action the 

 ether remains perfectly stiil This stillness 

 of the ether is what we call darkness, which 

 corresponds, as already stated, with a dead 

 level in the case of water. 



It was said in our first lecture, with refer- 

 ence to the colors produced by absorption, 



