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XXIII. Fish-Eye Views, and Vision under Water. By R. W. 

 Wood, Professor of Experimental Physics, Johns Hopkins 

 University*. 



[Plate III.] 



TN discussing the peculiar type o£ refraction which occurs 

 when light from the sky enters the surface of still water, 

 it seems of interest to ascertain how the external world appears 

 to the fish. As is well known, a submerged eye directed 

 towards the surface of still water sees the sky compressed into 

 a comparatively small circle of light, the centre of which is 

 always immediately above the observer, the appearance being 

 as if the pond were covered with an opaque roof provided 

 with a circular aperture or window. If our eyes were adapted 

 to distinct vision under water, it is clear that the objects sur- 

 rounding the pond, such as trees, horses, fishermen, &c, would 

 appear round the edge of this circle of light. Objects not much 

 elevated above the plane of the water would be seen somewhat 

 compressed and distorted, but the circular picture would con- 

 tain everything embraced within an anode of 180° in every 

 direction, i. e. a complete hemisphere. Our eyes behave so 

 abominably under water, however, that we can see nothing 

 of this curious picture ; the focus being so poor that it is 

 difficult even to distinguish between a piece of blank paper 

 and paper with black letters an inch high and a quarter of an 

 inch wide, the letters being almost invisible. . A lens of half 

 an inch focal length, placed in front of the eye, helps matters, 

 but even with this, vision is far from distinct. A great many 

 years ago I attempted to get an idea of how this circular 

 picture appeared to the fish, by constructing a diving-bell 

 out of an old butter firkin, weighted with lead, and provided 

 with a plate-glass window, forgetting that the refraction of 

 the rays from water to air again would banish the illusion of 

 the round window. 



It occurred to me last autumn during a lecture, that an 

 excellent notion of how we appear to the fishes could be 

 obtained by immersing a camera in water, and photographing 

 the circle of light, for it is easv enough to arrange a lens 

 and plate so as to obtain a sharp image when both are 

 immersed in water. The apparatus was constructed out of a 

 lard pail, a short focus lens, provided with a very small 

 diaphragm, being cemented over a hole perforated in a metal 

 disk which rested on a rim soldered around the inside of the 

 pail. The plate was placed on the bottom of the pail, and 

 the whole filled with clean water in a dark room. The lens 



* Communicated by the Author. 



