158 



NA TURE 



{Jan. I, 1874 



College of Science, and similar institutions in other dis- 

 tricts, all who are in any way connected with our arts and 

 manufactures may be trained to work on a method so 

 really scientific that Britain shall in this, as she certainly 

 is in some other respects, be foremost among the 

 nations. 



REFRACTION OF LIGHT MECHANICALLY 

 ILLUSTRATED 



IN preparing an elementary lecture on Light, intended 

 to be given at the Taunton College School, I have 

 had to consider how best to explain the somewhat abstruse 

 principle of optical refraction. It is true that Sir John 

 Herschel, in the sixth of his " Familiar Lectures on 

 Scientific Subjects," giving the explanation of refraction on 

 the undulatory theory, describes it as being " exceedingly 

 simple." The fact is, however, that it involves concep- 

 tions of wave-motion, difficult for any but advanced 

 students, and even they feel grateful to the eminent 

 physicist for the help afforded by a familiar illustration 

 with which he follows it. He desires his readers to 

 imagine a line of soldiers marching across a tract of 

 country divided at a straight boundary into two regions, 

 the one level ground suited for marching, the other rough 

 and difficult to walk over. Now if the line of soldiers 

 march with their line of front oblique to the boundary, 

 the men on the side just engaged in the heavy ground 



-/Vy.^ 



will be retarded as soon as they cross into it, so that if 

 the line be kept unbroken, the consequence must be a 

 change of front, which will leave the whole body of men 

 marching across the heavy ground in a new direction^ 

 in a word, their direction of march will have been re- 

 fracted. Now the light-waves emitted from a radiant 

 point being compared to the circles spreading from a 

 stone thrown into a pond, it is easily understood how a 

 sensibly straight portion of such a light-wave, passing 

 obliquely from one medium to another of different resist- 

 ance, will be refracted in a new direction. This simple 

 conception of change of front is at once apprehended by 

 the learner, to whom refraction thenceforth ceases to be a 

 molecular mystery, and becomes an intelligible mechani- 

 cal act dependent on the resistance of the two media 

 and the form of their limiting surface. Probably no point 

 in all Herschel's lectures has fixed itself in the memory 

 of so many intelligent readers. 



In following up the train of thought started by Sir 

 John Herschel's comparison, it occurred to me that an 

 instrument made to perform refraction mechanically 

 would be useful in teaching optics, and that such a con- 

 trivance would only require a pair of wheels running on 

 a table, into and out of a resisting medium. After a 



number of trials, made with the help of Mr. R. Knight, 

 a simple arrangement has been completed, which answers 

 satisfactorily in showing the behaviour of a ray of light 

 under the various circumstances of ordinary refraction. 

 Pieces of a thick-piled velvety plush known as " imitation 

 sealskin" are cut out to represent the sections of a thick 

 plate, a prism, a convex and a concave lens, and glued on 

 to smooth boards. The runner consists of a pair of box- 

 wood wheels mounted loosely on a stout iron axle, and is 

 trundled across the board, or still better, the board itself 



is tilted up, and the runner let go in the proper starting 

 direction. The following figures show the path of the 

 wheels, always from right to left of the page. 



In Fig. I, the runner starting from A, enters the rec- 

 tangle of velvet at B, where its left wheel being first re- 

 tarded, it shifts round into the direction BC, till it reaches 

 C, where the left wheel first emerging gains on the right, 

 so as to bring back the runner to the ultimate direction 

 CD. This illustrates the refraction of a ray of light in 

 entering and quitting parallel plane surfaces of a re- 

 sisting medium, such as a plate of glass. When the run- 

 ner enters at right angles to the boundary, its direction 

 is of course unchanged, as with the ray of light. 



Fig. 2 shows the path ABCD of the runner across a 

 triangle, corresponding with the course of a ray traversing 

 a prism. Also, by causing the runner to enter at about a 

 right angle near E, a direction is given to it which, if the 

 surface of the board and the triangle were similar as to 

 resistance, would make it emerge near F, at a small angle 



to the side. But the left wheel passing on to the smooth 

 surface gains so much on the right wheel still in the 

 velvet, that the axle slews round, the left wheel re-enters th 

 velvet, and the runner goes off in the direction FG, thus 

 illustrating the total reflexion which takes place when a 

 ray of light is directed to emerge very obliquely from a 

 more into a less resisting medium, as from a glass prism 

 or a surface of water into air. 



The action of the double-convex lens in causing parallel 

 or divergent rays to converge is shown by the path of the 

 runner in Fig. 3, which requires no further explanation, 

 nor does that corresponding to the divergent action of 

 the double-concave lens, Fig. 4. By starting two runners 



