252 OBSEKVATIONS OK VISION. 



directly the foil appears alwaj^s red and shar])ly defined; l)ut ob- 

 served obliquely it appears brighter, loses its color and takes on a 

 whitish brilliance, at the same time losing sharpness of contour. 

 These apparent changes of color and brightness can be noticed even 

 at a bright-red glow. 



iii. experiment with three olowlamps having lar(;e filaments 

 (''gray glow" and "red glow")." 



(a) The competition l)etween the two organs of sight is yet more 

 obvious when several parallel glowlamps with large filaments are 

 set u]) at a distance of 1 to Ik m. apart and provided with means for 

 gradually decreasing the electric current. As long as the Ijrightness 

 of the lami)s is so feeble that the rods assist in ol^serving them, only 

 the one directly observed appears red, while the others seem to ha^'e 

 a colorless, magical, rod-white luminosity. However quickly the eye 

 passes from one lamp to another, only that directly observed at the 

 instant is red — the others at once change to white. 



(7j) In order to show that the "gray glow'' is perceived before 

 the " red glow " appears, the current is entirely cut off. Then the 

 lamps fade slowly (owing to their thick filaments) and the directly 

 observed lamp, Avhich is red, disappears, while the others, seen 

 obliquely, still jiresent a white luminosity. By turning the current 

 off and on this obsi'rvation may be repeated as uuiny times as one 

 pleases, and the thicker the filaments the more successful the exjjeri- 

 ment. For a lecture experiment the distance of the laini)s would 

 naturally be greater. 



IV. EXPERIMENT WITH A VERY S>IALL RLUE FLAME. 



Observe directly and then obliquely a blue gas fiame from an easily 

 regulated l)urner with a small orifice. As observed directly the flame 

 api)ears sharply defined and of a blue color, but Avith indirect vision 

 its appearance changes to a disk of some size having the appearance 

 of moonlight and surroundc^l by a feebly luminous zone. 



v. COLORLESS APPEARANCE 0¥ THE SPECTRUM WHEN rilE ILLUMINATION 



IS FEEBLE. 



A ])ure si)ectrum of an arc light is produced, and its intensity 

 gradually diminished by means of two Nicol prisms. The red and 

 blue colors are the first to disappear, then the yellow, and finally 

 there remains in place of the bright -colored s})ectrum a colorless, dull 

 spectrum, which when viewed oblicpiely takes on a rod-white lumi- 

 nosity over its whole length. 



«(). T.nniiiier, Verb. il. D. Pliys. des. 1(5, 121-127, 1897: Wicd. Ann. (;2, 14-2!). 

 1897. 



