178 



NATURE 



[June 25, 1891 



sun. Those are, of course, conclusions which a very re- 

 stricted study of physical science will make perfectly 

 clear : why you get the two reds together when two 

 bows are visible ; why the blue is inside, and the red out- 

 side the single bow, also follows from a demonstration 

 which your teacher will give you, or which you can get 

 from a book. The main point is that a rainbow is 

 produced by a physical cause ; so that, if you once grasp 

 the idea of the cause of a rainbow, its whole anatomy 

 will remain for ever with you. 



It is quite impossible for you to see a rainbow in pro- 

 spective, or projected on the sky as an ellipse. That will 

 be quite clear, I think. Still, both these are recog- 

 nized art-objects. I am sorry to say that in this 

 year's Academy there is one case in which you will find 

 that the fundamental condition of having your back to 

 the sun has been neglected or forgotten by the artist. 

 In No. 395 a most exquisite stump of rainbow is seen, 

 most beautifully painted, and you naturally think, of 

 course, that you have your back to the sun, but the artist 

 has not been contented with painting the rainbow, he 

 has painted cattle as well, and their shadows sweep across 

 the picture. Another rainbow, 595, is excellently painted. 

 The artist not only knows a great deal about rainbows, 

 but wishes you to know that he knows, an umbrella being 

 emphatically en evidence. 



{To be continued^ 



THE FARADAY CENTENARY. 



ON Wednesday, June 17, at the Royal Institution, 

 Lord Rayleigh delivered a lecture in connection 

 with the hundredth anniversary of Faraday's birth. The 

 Prince of Wales presided. 



Lord Rayleigh said that the man whose name and 

 work they were celebrating was identified in a re- 

 markable degree with the history of that Institution. 

 If they could not take credit for his birth, in other 

 respects they could hardly claim too much. During 

 a connection of fifty-four years, Faraday found there 

 his opportunity, and for a large part of the time his 

 home. The simple story of his life must be known to 

 most who heard him. Fired by contact with the genius 

 of Davy, he volunteered his services in the laboratory of 

 the Institution. Davy, struck with the enthusiasm of the 

 youth, gave him the desired opportunity, and, as had been 

 said, secured in Faraday not the least of his discoveries. 

 The early promise was indeed amply fulfilled, and for a 

 long period of years by his discoveries in chemistry and 

 electricity Faraday maintained the renown of the Royal 

 Institution and the honour of England in the eye of the 

 civilized world. He should not attempt in the time at 

 his disposal to trace in any detail the stepsof that wonder- 

 ful career. The task had already been performed by able 

 hands. In their own Proceedings they had a vivid 

 sketch from the pen of one whose absence that day was 

 a matter of lively regret. Dr. Tyndall was a personal 

 friend, had seen Faraday at work, had enjoyed oppor- 

 tunities of watching the action of his mind in face of a 

 new idea. All that he could aim at was to recall, in a 

 fragmentary manner, some of Faiaday's great achieve- 

 ments, and if possible to estimate the position they held 

 in contemporary science. 



Whether they had regard to fundamental scientific 

 import, or to practical results, the first place must un- 

 doubtedly be assigned to the great discovery of the 

 induction of electrical currents. He proposed first to 

 show the experiment in something like its original form, 

 and then to pass on to some variations, with illustrations 

 from the behaviour of a model, whose mechanical proper- 

 ties were analogous. He was afraid that these elemen- 

 tary experiments would tax the patience of many who 

 heard him, but it was one of the difficulties of his task 



NO. I 1 30, VOL. 44] 



that Faraday's discoveries were so fundamental as to have 

 become familiar to all serious students of physics. 



The first experiment required them to establish in one 

 coil of copper wire an electric current by completing the 

 communication with a suitable battery ; that was called 

 the primary circuit, and Faraday's discovery was this : 

 That at the moment of the starting or stopping of the 

 primary current, then, in a neighbouring circuit, in the 

 ordinary sense of the words, completely detached, there 

 was a tendency to induce a current. He had said that those 

 two circuits were perfectly distinct, and they were distinct 

 in the sense that there was no conducting communication 

 between them, but, of course, the importance of the 

 experiment resided in this — that it proved that in some 

 sense the circuits were not distinct ; that an electric 

 current circulating in one does produce an effect in the 

 other, which is propagated across a perfectly blank space 

 occupied by air, and which might equally well have 

 been occupied by vacuum. It might appear that that 

 was a very simple and easy experiment, and of course 

 it was so in a modern laboratory, but it was otherwise 

 at the time when Faraday first made it. With ail his 

 skill, Faraday did not light upon truth without delay 

 and difficulty. One of Faraday's biographers thus 

 wrote: — " In December 1824, he had attempted to obtain 

 an electric current by means of a magnet, and on three 

 occasions he had made elaborate and unsuccessful at- 

 tempts to produce a current in one wire by means of a 

 current in another wire, or by a magnet. He still per- 

 severed, and on August 29, 1831 — that is to say, nearly 

 seven years after his first attempts — he obtained the 

 first evidence that an electric current induced another in 

 a different circuit. On September 23rd, he writes to a 

 friend, R. Phillips : I am busy just now again with elec- 

 tro-magnetism, and think I have got hold of a good thing, 

 but cannot say ; it may be a weed instead of a fish that, 

 after all my labour, I at last haul up." We now know that 

 it was a very big fish indeed. Lord Rayleigh proceeded 

 to say that he now proposed to illustrate the mechanics of 



the question of the induced current by means of a model 

 (see figure), the first idea of which was due to Maxwell. 

 The one actually employed was a combination known 

 as Huygens's gear, invented by him in connection with the 

 winding of clocks. Two sirnilar pulleys, A, B, turn upon 

 a piece of round steel fixed horizontally. Over these is 



