Prof. Dove on the relative Intensity of different Colours. 247 



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

 to oscillate upon the plane which bears it. 



That yellow and red colours 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 ex- 

 periments. But with these well-known phsenomena, another 

 stands apparently in complete contradiction. It has often oc- 

 cui'red to M. Dove, on quitting a picture gallery on the approach 

 of night, when he happened to cast a parting glance upon the 

 paintings, the red colour had altogether 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 ap- 

 plying 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 inin 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 thickness 

 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 out- 

 line, the blue alone is observed, as projection, upon its proper leaf. 

 If two red glasses be now placed before the openings of the ste- 

 reoscope, nothing whatever is seen ; while with two blue glasses 

 the relief appears in blue hues, and remains distinctly visible for 

 a quarter of an hour longer. Thus the fact of the earlier disap- 

 pearance 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 uniformly repeated. On this account the string of the 

 contra-basso must have a wider amplitude than that of the violin, 

 inasmuch as the diminished 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 deep voice of the seaman, strength- 

 ened by the speaking trumpet, is lost in the storm, the shrill 

 pipe of the boatswain still pierces through the howl of winds and 

 roar of waves. Savait has shown, by means of the toothed- 

 wheel, that the limit of sensibility of the car for grave tones is 

 extended by strengthening the strokes. The complete similarity 



