STUDYING THE HABITS OF FISHES. 



II2I 



Special uses, as on the coral reefs or where it is desired to photograph a consid- 

 erable area of the bottom from a considerable distance. 



The reflecting water glass designed b}^ the writer (1909) offers some advan- 

 tages over the ordinary form of water glass for certain kinds of work. It is a 

 rectangular box of galvanized iron (fig. 3, pi. cxv), about 2 feet long and 6 inches 

 by 8 inches at the ends, which are closed. Within the box at each end, as shown 

 in accompanying diagram, is a mirror placed at an angle of 45 degrees with the 

 long axis of the box and firmly fixed in a metallic setting. The reflecting surfaces 

 of the two mirrors are parallel and directed toward each other. The box is heavily 

 weighted with lead at one end so that when placed in water it floats in an upright 

 position, with about 10 inches of the 

 upper end projecting above the surface. 

 Opposite the lower mirror in the side of 

 the box is an opening filled by plate glass 

 bedded in aquarium cement (fig. 3, pi. 

 cxv) . Through this window light enters 

 from objects outside the box and these 

 objects are reflected in the lower mirror. 

 At the upper end of the box, on the side 

 opposite the first window, is an opening 

 through which the observer may look at 

 the surface of the upper mirror, and in 

 this mirror he sees reflected the surface 

 of the lower mirror with the objects on 

 the outside of the box shown in it. 

 The observer may thus stand upright in 

 the water, holding the water glass in front 

 of him, and by looking into the upper 

 mirror see submerged objects as he would 

 see them if his head were beneath the 

 surface. He sees the submarine land- 

 scape as it appears to a fish or to a diver through the glass window of his casque 

 A handle soldered to either side of the box enables the operator to turn it in any 

 direction and to hold it steady. Into the opening at the upper end of the box there 

 may be fitted a plate of metal to which are attached two tubes lined with chamois 

 skin and of such a size that the objective end of a pair of field glasses fit snugly 

 into them (fig. 3 , pi. cxv, right) . By inserting field glasses into these tubes the ob- 

 server may examine with them the objects shown in the upper mirror. Except 

 for the limits set by the opacity of the water, fish may thus be studied from a 

 distance as birds are studied in air. It should be added that the use of field 

 glasses is rarely necessary with this form of apparatus, as the observer is usually 



Fig. I. — Longitudinal section of the reflecting water glass 

 designed by the writer, w. Window on one side of the 

 submerged part; m, mirrors; c. cover over the upper 

 opening; /. tube for field glasses ; Id, lead weight. The 

 dotted line with the arrowheads shows the course of light 

 from an external object to the eye. The line a — b rep- 

 resents the water's surface. 



