166 ANNUAL OP SCIENTIFIC DISCOVERY. 



It is known that a green spot on a red field which is moved quick- 

 ly hither and thither, appears to oscillate. Wheatstone has shown 

 that a red heart on blue ground appears to oscillate still more quickly ; 

 hence the appearance is not to be referred to the action of comple- 

 mentary colors, but to a difference of refrangibility. Sir David 

 Brewster was the first to observe on geological maps, that blue and 

 red do not appear in the same plane ; and the reason of this M. Dove 

 considers to be rendered completely evident by his stereoscopic exper- 

 iments. His explanation of the fluttering heart is as follows : 

 When the sheet is moved in its own plane, the heart and the ground 

 on which it rests, describe tangents of the same absolute length, but 

 with radii which the eye regards as different. The angular velocities 

 of both thus appear to be different, and hence the object seems to oscil- 

 late upon the plane which bears it. 



That yellow and red colors approach the nature of light more than 

 blue, is an idea which may be traced throughout antiquity. In the 

 common language of the Germans, this is expressed by the terms 

 " screaming yellow," " burning red," in contradistinction to " deep 

 blue." This notion is corroborated by photometric experiments. But 

 with these well-known phenomena, another stands apparently in 

 complete contradiction. It has often occurred to M. Dove, on quit- 

 ting a picture gallery on the approach of night, when he happened 

 to cast a parting glance upon the paintings, the red color Had alto- 

 gether disappeared, while the blue appeared in all its strength. Artists 

 are well aware of this fact ; at least, on questioning such, M. Dove 

 has always found his own observation corroborated. 



The stereoscopic experiments already described, furnish an accurate 

 and beautiful method of observing this fact. On applying two glasses, 

 one of which permits the homogeneous blue rays to pass, and the other 

 the homogeneous red ones, the relief, as already stated, appears with 

 beautiful edges of red and blue lines which run alongside each other. 

 Although when the light is intense the red lines appear much the most 

 vivid, the blue glass made use of being more than ten times the thick- 

 ness of the red one, still as the twilight advances, the red becomes 

 weaker and weaker ; it finally disappears altogether, and instead of the 

 relief formed by the combination of the red and blue outline, the blue 

 alone is observed, as projection, upon its proper leaf. If two 

 red glasses be now placed before the openings of the stereoscope, 

 nothing whatever is seen ; while with two blue glasses the relief ap- 

 pears in blue lines, and remains distinctly visible for a quarter of an 

 hour longer. Thus the fact of the earlier disappearance of the red 

 rays is placed beyond a doubt : how is this to be accounted for ? 



It is known that weak impressions on the organs of sense singly may 

 arouse no consciousness, but do so where they are quickly and uni- 

 formly repeated. On this account the string of the contra-basso must 

 have a wider amplitude than that of the violin, inasmuch as the di- 

 minished number of vibrations demands a greater energy to render 

 them heard. Thus also if we wish to make ourselves heard without 

 great effort, we speak in a higher tone ; and hence it is that when the 



