252 OBSERVATIONS ON VISION. 



(liivclly the foil iip])t'ars ahvays red and sharply defined; but ob- 

 served obliquely it a])peai*s briii;hter, loses its color and takes on a 

 Avhitish brilliance, at the same time losing shai-pness of contour. 

 These apparent changes oH color and brightness can be noticed even 

 at a bright-red glow. 



JIT. EXPERIINIENT WITH THKEE CLOWLAMPS HAVINf! I.ARliE FILA:\rE:NT8 



("gray (;T.<nv •' and "red geoav")." 



(a) The competition l)et\veen the two organs of sight is yet more 

 obvious when several parallel glowlamps with large filaments are 

 set up at a distance of 1 to li m. apart and provided with means for 

 gi'adually decreasing the electric current. As long as the Ijrightness 

 of the lamps is so feel>le that the rods assist in observing them, only 

 the one directly observed aj^pears red, while the others seem to have 

 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. 



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

 the " red glow " apj^ears, the current is entirely cut off. Then the 

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

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

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

 off and on this observation may be repeated as many times as one 

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

 ment. For a lecture experiment the distance of the lamps would 

 naturally be greater. 



1\. experiment A\ nil a very small ULUE FLAME. 



Observe directly and then ol)lique]y a blue gas flame from an easily 

 regulated burner with a small orifice. As observed directly the flame 

 appears sharply defined and of a blue color, but with indirect vision 

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

 of moonlight and surrounded by a feebly luminous zone. 



v. COLORLESS APPEARANCE OI' THE SI'ECTRl .M AVIIEX THE ILLUMINATION 



JS KEE15LE. 



A i)ure spectrum of an arc light is pro(hiced, and its intensity 

 gradually diminished by means of two Nicol j)risnis. 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 vieA\'ed <)bli(|u('ly takes on a rod-white hnni- 

 nosity over its whole length. 



«0. Luiiinier, Verli. d. I). Pbys. Ges. It;, 121-127. 18!»7: Wied. Ann. 02, 14-20, 

 J 897. 



