30 REPORT— 1861. 



matical solid, the one drawn on a white, and the other on a dark gi'ound, were in- 

 serted in the stereoscope, the solid in relief appeared with a particular lustre. Prof. 

 Dove described the lustre as metallic ; and in another place, where he described the 

 two diagrams as di-awn, the one with white lines on a black ground, and the other 

 with black lines on a white gi'ound, he stated that the pyi'amid in relief " appears 

 lustrous, as made of gi'aphite." Other obsei-vers described the lustres dififerently, 

 some as resembling gi-oimd glass, and others as like paper darkened with a black- 

 lead pencil, while Professor Rood regarded it as "recalling the idea of highly 

 polished glass." In order to explain this phenomenon, Professor Dove remarked 

 " that in every case where a siu-face appeared lustrous, there was always a transpa- 

 rent, or ti-ansparent-refiecting stratum of much intensity, through which we see 

 another body. It is therefore externally reflected light in combination with inter- 

 nally reflected or dispersed light, whose combined action produced the idea of lustre. 

 This eflect," he elsewhere added, "we see produced when many watch-glasses are 

 laid in a heap, or when a plate of ti-ansparent mica or talc, when heated red-hot, is 

 separated into multitudes of thin layers, each of which, of inconceivable thinness, 

 is found to be highly ti-ansparent, while the entire plate assvmies the lusti-e of a 

 plate of silver." To these examples of lustre, produced by thin plates not in optical 

 contact, or if in actual contact, having difierent reflective powers, were to be added 

 the following pearls, mother-of-pearl, pearl-spar, and composite crystals of calca- 

 reous spar, and decomposed glass of all colours. The cause of these various kinds 

 of lusti-e, iind of that of metals, had always been well known, and when binoculai' 

 lustre attracted the attention of philosophers, it was natm'al to ascribe it to the 

 same cause. Professor Dove did this, and considered the dark surface in the one 

 picture as the dispersed light, and the white surface as the regulai-ly reflected light, 

 the dai-k surface beuig seen through the white sm-face. This theory of binocular 

 lusti'e, he had reason to believe, was not satisfactory. The phenomenon was first 

 obsened by himself in 1843, under conditions of different foiins than those under 

 which it was subsequently seen in the stereoscope. Having adverted to a paper 

 "On the knowledge of Distance given by Binocular Vision," published by himself 

 in 1844 in the ' Edinburgh Transactions,' he said that with his knowledge of the 

 phenomena he could not adopt Professor Dove's explanation of the lusti'e seen in 

 the stereoscope by the imion of figures on dai'k and white, or differently colored 

 surfaces. In order to test this explanation by other means, he combined surfaces 

 that had no geometrical figiu'es upon them, and he found that binocular lustre was 

 not produced. This experiment seemed decisive of the question. He was led to 

 infer from it that the lustre observed in the combination of right and left eye figures 

 of solids was not due to the rays from a dark surface passing through a lighter one 

 to the eye, but to the effect of the ej'es in combining the two stereoscopic figiires, 

 and to the dazzle occasioned by the alternating intensities of the two combined 

 tints, the impression of one of the tints sometimes disappearing and reappearing. 

 He refeiTed to an article published by Professor Rood, of Troy, on his (Sir David 

 Brewster's) " Theory of Lustre," and which he disavowed, not having adopted any 

 " theoiy of lusti-e." He had merely stai-ted an objection to Professor Dove's theory 

 of binocular lustre, and given an opinion regarding its cause ; and as the simple 

 experiment on which he founded that opinion had been made by others with a dif- 

 ferent result, he thought it right to re-examine the subject with the assistance of 

 other eyes than his own, and had obtained results which might be of use to those 

 who were disposed to study the subject more elaborately. 



Binocular lustre was a species of lustre sui generis. It was a physiological, not a 

 physical phenomenon, and had no I'elation whatever to those varieties of lustre 

 which arose from the combination of Lights reflected from the outer and inner sur- 

 faces of laminated, ti'ansparent, or translucent bodies. He assigned vaiious causes 

 for the physiological character of the phenomenon, and then added, " K binocular 

 lustre arises fi-om a physiological and not fi-om a physical cause, we must look for 

 this cause in the operations which take place in the eyes of the observer when 

 binocular lustre is distinctly seen. These operations are of two kinds. First, in 

 combining geometi-ical or other figures to represent solids whose parts are at dif- 

 ferent distances from the eye, the optic axes are in constant play, not only in vary- 

 ing the distance of their focus of convergence, to unite similar points at different 

 d'stauces in the two diagrams, but in maintaining the unity of the picture by 



