May, 1905 ] 



KNOWLEDGE & SCIENTIFIC NEWS. 



9^ 



be distinct enough below still water become indistinct 

 or invisible if the water be disturbed. To obviate this 

 difficulty it has become customary to make use of a 

 very efficacious and useful instrument called a water 

 telescope, which need be nothing more than a large 

 tube, say a foot in diameter, and say six feet long, 

 closed at one end with a sheet of glass. This end is now 

 plunged beneath the troubled surface of broken water, 

 and the observer applying his eye at the open end is 

 at once able to see as clearly as if the water were un- 

 ruffli-d, as, indeed, to his eve it now is. 



But let us pass on to consider how it is that the 

 surface of a transparent medium when broken up re- 

 fuses to allow rays of light to have free passage. Let 

 us take the case of a piece of clear glass, lying on a 

 newspaper, the printed matter of which is then seen 

 with perfect distinctness. But now commence pound- 

 ing up the glass with a hammer and you find that the 

 more completely the glass is broken up the more is 

 the printed page obscured, and when at last the glass 

 has become mere fine powder, it appears as a white 

 mass like so much salt, and nothing is seen behind it. 

 The fact is that light cannot penetrate the mass, be- 

 cause each ray as it passes from fragment to fragment 

 glances hither and thither off a myriad minute sur- 

 faces, and thus wastes itself in a multitude of reflec- 

 tions. In scientific parlance the optical continuity is 

 broken and the mass of powdered glass looks white 

 simply because it only reflects back the white light of 



day. It would have appeared just as white had the 

 glass been coloured, or even black. In the same way 

 and for the same reason the frolh on a glass of 

 Guinness's stout, instead of being dark brown, appears 

 white or nearly so. 



We now grasp how it is that without a water tele- 

 scope it is difficult to see through the surface of 

 troubled water at close quarters, but the fact which 

 we illustrate yet remains, namely, that when the eye 

 is removed to a distance the distraction caused by the 

 broken light largely disappears, and objects below are 

 seen more clearly. Another example strictly analogous 

 of the same sort of thing is afforded by either cloud or 

 mist. Cloud is simply composed of particles of water 

 mingled with particles of air, and though both 

 separately are perfsctly transparent, confused together 



they form a mass which stops and reflects back the 

 light, and for the same reason the illumined surfaces 

 of clouds are white, but in actual fact the stoppage of 

 light is not so complete as it appears, and a thin veil 

 of mist will behave precisely as the broken water's 

 edge, obliterating the view at short range, but to a 

 more distant observer allowing objects to be seen 

 through it with tolerable distinctness. Thus it often 

 happens that a balloonist whose view of the outside 

 world is wholly obscured by a shroud of thin mist can 

 be quite clearly seen by those at a distance. 



It should then be only in accordance with theory 

 and known fact if the secrets of the sea depths, which 

 hide themselves even from the trained eye of the sailor 

 on board ship, should become revealed to an aeronaut 

 who will poise himself in space overhead, say 10 times 

 higher than the maintop. 



It scarcely needs the further pointing out that there 



