156 ANNUAL OF SCIENTIFIC DISCOVERY. 



EXPERIMENT IX BINOCULAR VISION, ELUCIDATING THE PRINCIPLES 



OF THE STEREOSCOPE. 



A correspondent of the Journal of the Franklin Institute, states that a famil- 

 iar experiment, illustrating the principle of the stereoscope may he made by 

 looking: into a mirror and concentrating the ocular axes upon any spot on 

 the surface of the fflass, of an equal elevation with the eyes. If no such spot 

 exists, it can readily be supplied by wetting a piece of paper or wafer the si/;e 

 of a pea. 



The reflected images of the eyes being as far behind the glass as the eyes 

 are before it, and equidistant from each other, the ocular axes concentrated 

 upon the surface of the mirror will, if produced, cross and precisely meet 

 them, producing in the centre of the forehead one large cyclopiau eye. 



CURIOUS OPTICAL PHENOMENON. 



At the Aberdeen meeting of the British Association, Sir David Brewster 

 exhibited a curious specimen of chalcedony, in the interior of which was ;i 

 landscape minutely depicted. The landscape was evidently produced by the 

 action of nitrate of silver, which had been insinuated through pores into the 

 interior of the chalcedony. The most curious fact, however, about the spe- 

 cimen was, that the landscape entirely disappeared after being kept some 

 time in the dark, but was restored again in a most distinct manner, after an 

 hour's exposure to the light. 



Acting upon the suggestion afforded by this specimen, he had induced a 

 lapidary in Edinburgh to try the experiment of introducing a figure into the 

 interior of a mass of chalcedony, by drawing it on a polished surface of the 

 stone with nitrate of silver. The attempt was wholly successful, and the 

 figure of a dog could be distinctly seen in the centre of the specimen. 



OX THE PRODUCTION OF LIGHT, AND THEORY OF COLOR. 



In a paper on the above subject, read before the British Association, 1850, 

 by J. Smith, Esq., of Perth Academy, the author stated that he was unable 

 satisfactorily to account for certain nativral phenomena connected with light 

 by reference to either of the commonly received theories. A scries of exper- 

 iments were consequently undertaken with the object of clearing up this 

 difficulty, and these experiments led to the conclusion that varieties of color 

 are produced by pulsations of light and shadow. in definite proportions for 

 each shade of color. In order to make this point clearer, let us suppose 

 white light to be caused by motion in a fluid, and black by the absence of 

 motion, then a certain color blue, for example would be produced by a 

 certain proportion of alternate rest and motion of this fluid. The following is 

 an account of some of the experiments to which Mr. Smith had recourse 

 during his investigations. He first caused a narrow parallelogram of card- 

 board to revolve over a black body with a rapiditv which he considered equal 

 to Ihe vibration* of light in a second of time. By this morion he obtained a 

 distinct blue, while at another time, in different weather, the same thing 

 produced a purple. He then made a disk, with several concentric rings, 

 which he painted respectively one-third, two-thirds, throe-quarters, and one- 

 half black, leaving the remainder white, and on making this disk revolve 

 rapidly, the rings became completely colored there was no longer any 



