164 ANNUAL OP SCIENTIFIC DISCOVERT. 



long-sighted eye sees the latter, and a short-sighted eye the former. 

 Hence the experiment furnishes us with a kind of optometer ; to 

 this purpose M. Dove has applied it in hundreds of cases, and 

 never found a single individual whose eyes fulfilled the conditions 

 of achromatism at all distances. Acquainted with this fact, and 

 observing a certain analogy between it and his stereoscopic exper- 

 iments, he naturally sought the cause of the phenomena presented 

 by the latter in the non-achromatic nature of the eye. 



A fine white line drawn upon a black ground was viewed through 

 the glasses used in the stereoscopic experiments. It was ascertain- 

 ed that, to be plainly visible, it must be held at a greater distance 

 from the eye when the red glass is used than when the blue glass 

 is applied. Sir David Brewster has obtained an analogous result 

 with pigments. A number of square pieces of gradually decreas- 

 ing size was cut from the same vividly-colored card, and placed 

 one upon the other so as to form a pyramid with ascending steps, 

 all of the same height. Two such pyramids were built, the one 

 beside the other ; the squares were blue and red ; one pyramid had 

 a blue square for its base, the other a red one. It was always 

 found that a blue square placed upon a red one appeared higher 

 than a red square placed upon a blue one ; so that in the building 

 of the pyramids, each appeared by turns to exceed the other in 

 height. From this experiment it follows, that at the distance of dis- 

 tinct vision the lines of convergence of both eyes enclose a smaller 

 angle in the case of red light than in the case of blue. Hence, if an 

 observer, who sees equally well with both eyes, have both colors pre- 

 sented, to him in the stereoscope in the manner already described, the 

 lines cannot coincide, but will project themselves in directions which 

 cross each other upon a surface which does not pass through the 

 point of intersection of both directions. 



M. Dove next goes on to discover the cause of the glistening, which 

 for example, is observed on the surface of varnished pictures, and 

 which may be destroyed by quenching the polarized rays with a 

 Nichol's prism. In every case in which a surface appears thus shi- 

 ning, there is a reflecting layer, more or less transparent, through 

 which another body is viewed : the glistening owes its origin to the 

 combination of the rays reflected from the surface and those which 

 pass through the transparent layer from the body behind. This is in- 

 creased when the number of the alterations of the layers increases. 

 Thus mica assumes a metallic lustre, and layers of glass plates 

 the appearance of mother-of-pearl. In the projection of a truncated 

 pyramid intended fora certain eye, the section was colored with a 

 saturated wash of blue ; in the figure intended for the other eye, the 

 section was colored yellow. At the moment of combination, when 

 the resultant green appeared, it seemed as if one layer of color had 

 become transparent and that the other was seen through it. When 

 the colored section was viewed through a violet glass held before both 

 eyes, the surface appeared like polished metal. 



These experiments are intimately connected with the phenomena 



