328 PETER GUTHRIE TAIT 



when the object is not far beyond the critical distance. Thus there may have been, 

 in all of Scoresby's observations (though he has only occasionally noticed and depicted 

 them) an erect image above each inverted one, but so much reduced in vertical 

 height as to have been invisible in his telescope, or at least to have formed a mere 

 horizontal line so narrow that it did not attract his attention. The greatly superior 

 number of inverted images, compared with that of the direct ones, figured by 

 Scoresby, may thus be looked upon as another independent confirmation of the 

 approximate correctness of the hypothetical arrangement we have been considering. 



To obtain an experimental repetition of the phenomena in the manner indicated 

 by the above hypothesis, a tank, with parallel glass ends, and about 4 feet long, 

 was half-filled with weak brine (carefully filtered). Pure water was then cautiously 

 introduced above it, till the tank was nearly filled. After a few hours the whole 

 had settled down into a state of slow and steady diffusion, and Vince's phenomenon 

 was beautifully shown. The object was a metal plate with a small hole in it, and 

 a lamp with a porcelain globe was placed behind it. The hole was triangular, with 

 one side horizontal (to allow of distinction between direct and inverted images) 

 and was placed near one end of the tank, a little below the surface-level of the 

 unaltered brine, the eye being in a corresponding position at the other end. A 

 little vertical adjustment of object and eye was required from time to time as the 

 diffusion progressed. The theoretical results that the upper erect image is usually 

 much less than the object, and that it is seen by slowly convergent rays, while the 

 inverted image is larger than the object and is seen by diverging rays, were easily 

 verified. 



To contrast Wollaston's best-known experiment with this, a narrow tank with 

 parallel sides was half-filled with very strong brine, and then cautiously filled up 

 with pure water. (The strong brine was employed to make up, as far as possible, 

 for the shortened path of the rays in the transition stratum.) Phenomena somewhat 

 resembling the former were now seen, when object and eye were nearly at the 

 same distance apart as before, and the tank about half-way between them. But in 

 this case the disparity of size between the images was not so marked the upper 

 erect image was always seen by diverging rays, the inverted image by rays 

 diverging or converging according as the eye was withdrawn from, or made to 

 approach, the tank. In this case, the curvature of each of the rays in the vessel 

 is practically constant, but is greatest for the rays which pass most nearly through 

 the stratum of most rapid change of refractive index. Hence, when a parallel beam 

 of light fell horizontally on the tank and was received on a sufficiently distant 

 screen, the lower boundary of the illuminated space was blue and the progress of 

 the diffusion could be watched with great precision by the gradual displacement of 

 this blue band. I propose to employ this arrangement for the measurement of the 

 rate of diffusion, but for particulars I must refer to my forthcoming paper. 



Wollaston's experiment with the red-hot poker was probably, his experiment 

 with the red-hot bar of iron almost certainly, similar to that above described with 

 the long tank, and the weak brine ; and not to that with the short tank, though 

 the latter is usually cited as Wollaston's contribution to the explanation of the 

 Vince phenomenon. We have seen how essentially different they are, and that the 

 latter does not correspond to the conditions presented in nature. 



