350 Mr. R. W. Wood on some Experiments 



may thus cross again and again, traversing a path which is 

 approximately a sine curve. 



While showing the curve-trajectory of a ray of light in a 

 vessel filled with brine, the density of which increased with 

 the depth, it occurred to me that by properly regulating the 

 refractive index of the liquid, the ray might be made to 

 traverse a sine curve. 



Some attempts in this direction were so successful, and 

 yielded such beautiful experiments for the lecture-room, that 

 it seems worth while to publish them, together with some 

 photographs of the trajectories, although, as I have since 

 learned, very similar experiments have already been described 

 by Wiener ( u Gekriimmte Lichtstrahlen," Wied. Annalen, 

 xlix. p. 105). 



For the liquid 1 adopted an arrangement very similar to 

 the one described by Prof. Everett for obtaining mirage in a 

 rectangular tank. 



A trough was first made of plate glass, about 50 cms. long, 

 10 cms. bigh, and 2 cms. wide. This was filled to the depth 

 of 3 cms. with a concentrated solution of alum. By means of 

 a pipette, of the form shown in PI. III. fig. 1, water containing 

 about 10 per cent, of alcohol was carefully deposited on the 

 alum solution to a depth of 3 cms. The addition of the alcohol 

 brings up the refractive index of the water, and is necessary 

 for reasons that will be spoken of presently. As a liquid of 

 high index, with a specific gravity intermediate between that 

 of the other two liquids, I used, instead of sugar and whiskey, 

 a mixture of glycerine and 85 per cent, alcohol, the right pro- 

 portions being easily found by experiment. 



The mixture should float on the alum solution and sink in 

 the water, and is introduced between the two layers by means 

 of the pipette at the end of the trough through which the ray 

 is to enter. A layer about 3 cms. in thickness will be found 

 about right. 



All three of the solutions should be first acidified with a few 

 drops of sulphuric acid, and rendered fluorescent with quinine. 

 The difference in surface-tension between the two upper 

 layers may give some trouble: when the pipette is withdrawn 

 it may draw a thread of the glycerine and alcohol mixture up 

 through the water, and a complete upsetting of the layers 

 occur as a result of the forces of surface-tension. 



This invariably happens when alcohol is not added to the 

 water, and can be remedied either by the addition of water to 

 the glycerine mixture or of more alcohol to the water. It is 

 a good plan in withdrawing the pipette to pull it out slowly 

 in a very oblique direction, in order that the heavy liquid may 

 be washed off before the tube reaches the surface. 



