19G 



♦ KNOWLEDGE ♦ 



[Sept. i, 1885. 



to walk to Braemar througli Glen Tilt, and was overtaken 

 I)}' a Highland lassie, a beautiful specimen of a noble race. 

 She had been the belle of the ball, and was engaged to 

 its hero, a stalwart Cameron, who had carried ofE all the 

 best prizes at the athletic competition. We walked on 

 together, but though an able pedestrian, I had a struggle 

 to keep pace with my vigorous barefooted companion. 

 Observing this, she suggested that I should cast off my 

 shoon and be free. I did so, and the ground being 

 favourable for a beginner, perceived at once that my 

 natural pace became the same as hers without the forcing 

 or strain that was previously necessary. In the course 

 of many thousands of miles of subsequent pedestrian 

 excursions I have frequently done the like with similar 

 advantage, though unable to continue a long distance on 

 account of the tenderness of artificially-swaddled soles. 

 Crossing a sandy bay at low tide is very tempting, and 

 if the sands are hard a curious result follows a long walk. 

 After a few miles a slight tenderness is felt in the ball of 

 the largo toe. This increases, and presently becomes 

 painful. On examination, the cuticle of that part is 

 found to be ground smoothly away by the friction on the 

 sand. My first experience of this was in crossing one of 

 the fine bays near the Giant's Causeway. Unconscious 

 of what was going on I persevered until the pain grew 

 serious, and then found that the cutis was bared. The next 

 day I was unable to walk at all. I name this as a warn- 

 ing to others who may be induced to try experiments in 

 barefoot walking. 



When bare feet are objectionably conspicuous, or the 

 ground severe, I walk on for an hour or two in orthodox 

 fashion, then stand ankle deep in a brook ; or pour water 

 in my shoes, and keep my feet scrupulously wet during 

 the rest of the day. This adds about 20 per cent, to the 

 possible mileage. One consequence of this, and the habit 

 of usually wearing very loose slippers, is that, though no 

 longer a boy, I laugh at the idea of suffering any evil 

 consequences from wet feet, whether walking, standing, 

 or sitting. 



OPTICAL RECREATIONS. 



Bi "A Fellow oi- xirE Royal Astronomical Society. " 

 (^Continued from Page 511 of Vol. VII.) 



IF," says Sir David Brewster in his Optics, "We 

 transmit a beam of the sun's light through a cir- 

 cular aperture into a dark room, and if we reflect it from 

 any crystallised or uncrystaUised body, or transmit it 

 through a thin plate of either of them, it will be reflected 

 and transmitted in the very same manner, and with the 

 same intensity, whether the surface of the body is held 

 ^bove or below the beam, or on the right side or left, or 

 on any other side of it, provided that in all these cases it 

 falls upon the surface in the same manner ; or, what 

 amounts to the same thing, the beam of solar light has 

 the same properties on all its sides ; and this is true, 

 whether it is white light or directly emitted from the 

 sun, or whether it is red light, or light of any other 

 colour. The same property belongs to light emitted from 

 a candle or self-luminous body, and all such light is 

 called common light." 



Now we have previously seen in the course of these 

 Essays (Vol. V., p. 352, et alibi) that light consists of a 

 series of undulations or vibrations transverse to the 

 direction in which it is propagated, and it is pretty 

 evident that these vibrations must occur symmetrically 

 as regards the axis of the beam. Let us suppose that 



. 44 the left-hand circle represents a section of a 

 of light coming through a small round hole into a 

 an will tiie vibrations of which it is 



Fig. 4i. 



made up be performed in the direction C D, as well as 

 in that A B. Or possibly an extremely simple model, 

 which the reader may make at once, will render our 

 meaning a little clearer. Let him, then, get a piece of 

 card, and with a penknife cut out two undulating strips 

 like V (Fig. 45). A horizontal slit in one of them will 



Fig. 43. 



enable the other to be inserted through it at right 

 angles, and we shall obtain the V^ of the same figure, 

 which will rejiresent our symmetrical beam of light — 

 symmetrical because it is obviously alike in every 

 direction. Is it possible in any way to alter this dis- 

 position of the vibrations of the ether, and so obtain a 

 beam with sides 1 Let us see. Many of our London 

 readers must recollect the shop of the late Professor 

 Tennant in the Strand, and the splendid rhomb of Ice- 

 land spar which used to be exhibited in the window, 

 showing a double image of a red wafer |ilaced behind 

 it. A description of this will form 

 to the study of the so-called " polavis; 

 to which we are now about to addres 

 rhomb, as most people are i 

 six equal and similar rhomboids- 

 defined by Euclid (Definition XXXIII.) as having 

 opposite sides equal to each other ; but all its sides are 

 not equal, nor its angles right angles." Well, whether 

 we find Iceland sjiar in a crystalline or massive condition 

 it will always split up into rhombs, like Fig. 46. 



of light," 



selves. A 

 s a solid bounded by 

 rhomboid being 



Fig. .10. 



The opposite sides of a crystal of this sort arc parallel, 

 and the line AX (known r.s the axis of the crystal) is 

 equally inclined to all its six faces at an angle of 45° 23'. 

 Suppose now that we obtain such a crystal (the larger it 



