10 REPORT 1865. 



the second eye cannot see the other picture. If, before the sensation of one eye 

 exhausted, the slid j shuts the first lens and opens the other, a new impression ispro- 

 duced, and we have an uninterrupted sensation of vision as if the object had moved 

 before as ; and if a sufficient number of pictures represent that object in the various 

 consecutive positions it has assumed during the several stages of its motion, we expe- 

 rience the sensations we hare when we see the object while moving; and although 

 the pictures in their limited number do not show all the intermediate positions, 

 still the mind has the power of filling up the deficiency, as it does if, when looking 

 at a real object in motion, we accidentally wink the eyelids, or an obstacle happens 

 to pass between us and the object. To exemplify this, Mr. Claudet has employed 

 two photographic pictures, one representing the beginning of an action, the other 

 the end. By moving the slide one way, the right eye can see the picture repre- 

 senting the figure in one position, while the picture showing the other is invisible 

 to the left eye ; then by moving the slide the other way, the left eye sees the figure 

 in the second position, and the first picture is invisible to the right eye. Although 

 we have really only seen the figure in two extreme positions, still we have the illu- 

 sion of having observed the intermediate positions — as, for example, in a slide 

 exhibited having one picture of a boxer with his arm close to his side as preparing 

 to hit, and another with the arm extended delivering the blow. Here, although all 

 the intermediate positions are omitted which must have been assumed during the 

 act, the miud completes the action. Another curious phenomenon of this alternate 

 vision is, that one cannot distinguish which eye the object is seen by ; for although 

 the vision is transferred alternately from one eye to the other, we are not conscious 

 of the act; and during the change of pictures which has taken place in the mean* 

 time, we have had a uniform and uninterrupted sensation, and consequently it has 

 appeared as if the object were moving. 



On Spectacles for Divers, and on the Vision of Amphibious Animals. 

 By F. Galton, F.B.S., F'.G.S. 

 Bathers who have surmounted the very natural repugnance, felt by beginners, to 

 open then.' eyes when they dive, find when they look about them under water that 

 nothing is to be seen with distinctness. They perceive little more than a haze of 

 diffused light ; for their eyes are thoroughly out of focus in a water medium. "When 

 a man under water holds his hands at a little distance from his face, so great is the 

 confusion of outline, that he cannot discover the spaces between his fingers even when 

 he has separated them as widely as possible. The appearance is a formless blurr of 

 white. Now what is the precise cause of this indistinctness of vision ? By what 

 optical arrangement can it be overcome':" And how do amphibious animals accom- 

 modate their sight to the requirements both of air and of water P Suppose a tube, 

 with a flat bottom of glass, tilled with water ; when the surface is perfectly still, 

 and we look down the tube, we see objects lying in the water and others in the air 

 below the glass bottom, with perfect distinctness. But if we bend the head down 

 to the tube, the instant the eye touches the water all distinctness of vision ceases. 

 The convex surface of the eyeball has indented the plain surface of the water and 

 thereby turned the tube into a concavo-plane water-lens. The convexity of the 

 eyeball is very great ; according to physiologists, the radius of its curvature is only 

 0'31 of an inch ; the effect of the concave lens which it stamps on the surface of 

 the water must be proportionately large ; and if it be desired to counteract its in- 

 fluence, a convex lens must be used of such high power that, when immersed in 

 water, its effect shall be equal and opposite to that of the concave water-lens. A 

 simple calculation shows the description of lens required. A double-convex lens of 

 dint glass, each of whose surfaces has a radius of 0'48, is the equivalent. It would 

 exactly neutralize the effect of the concave water-lens, if it were held close to the 

 eyeball. This curvature of the lens would require to be somewhat modified accord- 

 ing to the convexity of each individual eye, and to the refractive power of different 

 kinds of tlint glass." When held at the usual distance of an eye-glass from the eye, 

 a lens of more moderate power, such as a radius of OGO, or even 070, is found 

 sufficient. Furnished with eye-glasses containing suitable lenses, we might expect 

 that the vision of a diver would be rend* red as (dear under water as in air, that its 

 range wotdd be limited only by the turbidity of the water, and that it would not be 



