KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1505 



Seeing BeneoLth the 



WSlVCS. 



By THE LATE Rev. J. M. Bacon. 



A CURIOUS controversy arose fifty years ago concern- 

 ing the old fable of the " Dog and the Shadow," which 

 opened up the subject of vision through water. On 

 the one side Doctor Lardner maintained that the story, 

 " handed down through so many ages, diffused through 

 so many languages, and taught so universally in the 

 nursery and the school," of a dog being able to see in 

 water the reflection of himself and the meat in his 

 mouth, " was a most gross optical blunder." On the 

 other hand, critics were not found wanting who im- 

 plied that the fable only represented a fact which 

 ought to be familiar in all possessing ordinary ob- 

 servation. Thereupon the doctor retaliated with an 

 experiment of his own, the futility of which should 

 hardly need pointing out. He filled a basin with water, 

 and, placing it near an open window, looked down 

 upon it from a height of five feet, and saw no trace of 

 his learned countenance therein. 



Now, had the doctor simply gone to his water butt, 

 the water within which we will suppose to be clear 

 and not too near the top, and looked in, being careful 

 also that sufficient side light illumined his features, he 

 could have seen quite well enough to shave himself, or 

 by holding a piece of printed paper downwards, not 

 quite squarely but a little slanting, so as to catch the 

 light, he could have had little difficulty in reading 

 any ordinary type. Circumstances would have still 

 further helped this experiment if a projecting roof or 

 tree had overhung, so as to somewhat moderate the 

 overwhelming light background of sky. From this it 

 will be seen that experimenting with a white shallow 

 basin near a window was a ridiculously unfair test of 

 the truth of the old story, whereas were he to have 

 stood over a deep dark overshadowed pool, which 

 might reasonably have been presupposed, he would 

 have found that the gross blunder was hardly in the 

 fable. 



But the controversy alluded to elicited some well- 

 established physical facts which supply the argument 

 of the present paper. It was first of all pointed out 

 that the image of the banks of a lake or river viewed by 

 an observer stationed at a considerable distance on the 

 onfiosite side are very vivid, but become less so if the 

 observer, being, we must suppose, in a boat, begins 

 to approach nearer, the reason being that " when a 

 ray falls so obliquely upon the surface of water as to 

 make with the surface an angle of 15°, nearly a fourth 

 of all the incident rays are reflected." 



All this, however, can be stated more simply and 

 intelligibly. -Ml the world knows the difficulty of 

 hitting any object under water with a shot gun. If 

 fired nearly perpendicularly downwards over the side 

 of a boat it is true that the shot will penetrate the 

 water fairly steadily and truly, but it is otherwise if 

 the gun has to be pointed in a slanting direction. In 

 this case the shots enter the water reluctantly, taking 

 only a shallow dive below, and in an extreme position 

 the shots will not enter the water at all, but be re- 

 flected sheer off the surface. 



It is practically the same with rays of light, and so 

 it comes about that the image of a distant bank, being 

 seen by rays which are very much aslant, and, there- 

 fore, very well reflected, is particularly vivid. And 



the converse of this is also true, thus — Imagine a suffi- 

 ciently distinguishable object, say a fish's eye, three 

 feet below clear water. This might be seen readily 

 from a position directly overhead, but less distinctly 

 if viewed at a slant angle, and actual experiment shows 

 that all rays from the fish's eye which are so aslant 

 as to reach the surface of the water beyond a radius 

 of four feet never get out of the water at all, but are 

 simply reflected back from the water's surface, which 

 in this case acts as a perfect mirror. Thus an ob- 

 server looking towards the fish from a position which 

 is outside this limit will not see the fish, nor — pace 

 certain fishermen I have known — will the fish see him. 

 To put this fact beyond dispute let the following ex- 

 periment be tried. Stand a tumbler nearly full of 

 water on an open newspaper near the edge of a table, 

 and then, placing your eye on a level with the table and 

 six inches from the tumbler, look aslant upwards at 

 the surface of the water. You then learn in a most 

 convincing manner that the water's surface allows no 

 outside rays to pass to your eye, but simply behaves 

 as a mirror, revealing the print of the paper \\ith the 

 most perfect reflection. 



We are now prepared to begin an enquiry into a 

 curious and all-important phenomenon which it fell to 

 the lot of the writer to be able to put to a crucial test. 

 It needs no pointing out th.nt in na\al warfare, as being 

 carried on at the present hour, there is nothing more 

 deadly or more to be dreaded than the snares which 

 are caused to lurk beneath the water — the mine, the 

 torpedo, and the submarine. It is of paramount im- 

 portance, therefore, to get, if by any means, some 

 inkling of all that may lie at a moderate distance be- 

 neath the water line, and it has long been known that 

 this may best be done by looking down into the water 

 from a considerable height overhead. Even in peaceful 

 navigation, when some danger, as, for instance, a shoal 

 or sunken wreck or the like, is suspected of lying in 

 a vessel's course, but cannot be seen from deck, then 

 it is customary to send a man aloft, and the higher in 

 reason that he can climb the further will his vision 

 penetrate, and the better will his eye command a view 

 of any submerged object. It was to determine the 

 full extent to which this fact could be turned to ac- 

 count that the writer was commissioned under the 

 auspices of the Admiralty to endeavour to obtain photo- 

 graphs of the sea bottom from a balloon. This feat 

 was actually accomplished during an aerial sail over 

 the Irish Sea from the Isle of Man, a voyage which 

 became historical, and which resulted in the securing 

 of a very remarkable photogr.iph of the sea 

 bed, showing varied rock and sand lying in 10 

 fathoms, that is 60 feet of water, and that water 

 strongly ruffled after a week of boisterous weather. 



Now it should be clear that the half of the secret of 

 success in such an attempt has been already told. 

 For if, say, a sunken vessel were lying in a few fathoms 

 of water, and a man were looking down on it from a 

 boat, and floating somewhere just over its middle part, 

 then that middle part might be fairly well seen, but 

 the more distant parts bfith fore and aft, being viewed 

 at a slant angle, would probably be altogetbiT invisible. 

 If, however, the observer were to be let up a quarter 

 of a mile into the sky, and to look down from there, 

 all parts of the vessel would now lie practically per- 

 pendicularly below, .'md all would be equally well seen. 



But in attempting to look beneath the water's surface 

 at sea there is another obstacle to be reckoned with, 

 and that is the usually troubled nature of that surface. 

 For it is an everyday experience that objects which may 



