(J. N. Rood on Stereoscopic Experiments, 201 



1 combined two such black surfaces as A and B (fig. S) ; the 

 ground was white. Now it is evident that when the edges c c 

 and hh coincide, dd cannot coalesce with iV, consequently the 

 narrow band, included by the dotted line, appears white to one 

 eye and black to the other ; in spite of this, however, the play 

 of the ocular axes was so rapid, and the two combinations so 

 quickly effected, that no lustre on tiiis band was perceptible. 

 Similar experiments with yellow squares on a blue ground gave 

 a like result. When, however, the inequality was in a vertical 

 direction, the unmatched band or edge showed lustre with dis- 

 tinctness. 



As Dove* has pointed out that the stereoscopic experiments 

 published by me in this Journal f still further confirm his posi- 

 tion in opposition to some distinguished physicists, that lustre 

 does not necessarily depend upon an idea of solidity or depth, I 

 will here mention two additional experiments, which would 

 seem to remove the last shadow of doubt on this point. I com^ 

 bined in the stereoscope black surfaces i inch square with various 

 tinted papers 1^ inches square, when lustre resulted, although 

 tbe two figures totally refused to unite into one. Finally, a 

 piece of red or white paper, 1 inch square, was pasted on one 

 side of a blackened stereoscopic slide, which was then placed in 

 the stereoscope. Here but one figure was present; to the other 

 eye merely a dark field was presented ; consequently, the diffi- 

 culty of uniting two stereoscopic pictures was not present. After 

 a little trial brilliant lustre was produced, as in the former case. 

 This experiment is somewhat more difficult to make than those 

 previously described by Dove and myself, simply because it is 

 tiere not quite so easy to divide the attention equally between 

 the two eyes. The lustre then is independent of the idea ot 

 oepth, and two figures are useful merely in enabling the observer 

 the more readily to pay equal attention to the two impressions. 

 A he following experiment points to the same conclusion. 

 3. Production of Lustre by the convergence of the Ocular Axes. 



In the following beautiful experiment the idea of solidity is 

 ^ot only excluded, but the lustre is seen detatched even from a 

 J]aterial surface. An aperture, i inch in diameter, is cut in a 

 Wackened piece of cardboard, and across the opening a sewing- 

 J^fdle is fastened by a little wax. The card thus prepared is 

 S at a distance of six or eight inches from the eyes, while 

 >«hind it, at a distance of a footer more, a sheet of ^^^^f . ^r 

 "^ placed beside one which is blackened. The cardboard is held 

 ^^ such a manner that the aperture appears black to one eye and 

 ^^ite to the other; the ocular axes are then converged on the 

 t Vol. »xi. May, 1861. 

 . XXXIV, No. IOL-Sept., 1862. 



