Prof. Dove on the Stereoscopic Combination of Colours. 2dt5 



reoscope in the manner already described, the lines cannot coin- 

 cide, 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 consider 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 NichoFs prism. In every case in which a 

 surface appears thus shining, there is a reflecting layer, more or 

 less transparent, through which another body is viewed ; the 

 glistening owes its oi'igin to the combination of the rays reflected 

 from the surface and those which pass through the transparent 

 layer from the body behind. This is increased when the number 

 of alternations 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 in- 

 tended for a certain eye, the section was coloured vdth a satu- 

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

 section was colom-ed yellow. At the moment of combination, 

 when the resultant gi*een appeared, it seemed as if one layer of 

 colour had become transparent and that the other was seen 

 through it. When the coloured section was viewed through a 

 violet glass held before both eyes, the surface appeared like 

 polished metal. 



These experiments are intimately connected with the phseno- 

 mena of irradiation. They establish the fact, that the deport- 

 ment of black and white towards the eye is exactly similar to that 

 of two different colours. The lustre obtained by the combination 

 of black and white is peculiarly strong ; so decided, indeed, that 

 some, and among others the writer of this report, compared it to 

 the lustre of lead glance or of tin, although the component white 

 and black were both perfectly dull and lustreless. According to 

 the explanation already given, one of these surfaces must appear 

 in advance of the other. The viewing of an object by the naked 

 eyes by different degrees of illumination with white light is ana- 

 logous to those experiments witli coloui-ed light, where the object, 

 to be distinctly seen, must be brought nearer in the case of blue 

 light than with red. A dark object will, under the same conditions, 

 appear farther off than a white one, as the red surface appears 

 more distant than the blue. At the distance of distinct vision, 

 the flame of a candle, when viewed through the violet glass, which 

 permits the ends of the spectrum to pass and extinguishes the 

 middle, appears violet ; that is, the red flame is as large as the 

 blue. At the distance of distinct vision, a white object also aj) • 

 pears of the same size as a black one ; at a greater distance, the 

 blue flame embraces the red ; that is, beyond the distance of 



