OUSKRVATIONS ON VISloX. 251 



condLicdon (coiitacl tlicorv). ^^'llil(' (lie t'liiiclic i of {\\c>v conliict 

 stations has beiMi coiiipan'd to dial of an clccli'ical iiilcri-ii])t()r. I am 

 iiicliiu'tl to think thai llu'iv is liciv inserted in tii'^ condiiftini:- path a 

 potential eapacity vduch incivases the seH'-indnct ion due t<» the i)r<)pa- 

 U'ation of the eU'ctro-niaiiiietic h^'ht waves, in re\('rse of the pi-ocess 

 of Piipin for telephoninii' ovei' u'reat distanees hy means of cahh's. 



C. Kxi'rJM.MEXTs ox "\^isi()x IX r>i;i(iirrxKss and ()i',s( rini-i'. 



In order to ilhisti'ate the functions of the rods and cones and their 

 competition, the foHowinii' e.xpei'imenfs are inti'o(hiceil : 



r. ni:\ KnoiwiKxr ok tiik spectkum uy (iUAnr.vr iikatix*; or ax ixcax- 



OESCRXT I. A:\rp. 



An incandescent hinip with hir^e hhunent is phu-inl in a suiiabh^ 

 box, and means are provided for gradually varying the heating cui-- 

 rent between the limits corresponding to dull-red glow and incandes- 

 cence. The filament serves as the source of light and a large spec- 

 trum is thrown on a white screen I)y means of a Rowland concave 

 grating. After the eye has become accustomed to complete dark- 

 ness the current is increased nntil the fii-st appearance of light is 

 i-ecognized on the screen. This first recognizable light is colorfess, 

 and makes the impression of '' rod-Avhite '" luminosity. As the cur- 

 rent is increased the intensity of the colorless light waxes and ap- 

 pears to stretch farther toward both ends of the si)ectrnm, and sensa- 

 tions of color are gradually associated with those of light. After 

 color begins to appear the interesting ob.servation is made that the 

 position of apparent maxinunn intensity, lying first in the blue green, 

 is displaced more and moic toward the yellow gi-een of the spectrum. 

 This displacement may be confirmed by marking the brightest spot 

 and then letting the lamp gradually wane in intensity till only gray- 

 white luminosity remains. The spot where this is seen brightest is 

 (hen marked, and the light is increased till both marks are seen. 



P^rom this experiment it follows that the rods fii-st perceive light, 

 and that their greatest sensitiveness is for the blue-given region of 

 spectrum, while the cones are most sensitive tt) yellow green. The 

 experiment is founded on the researches of Draper " and II. F. Weber.'' 



II. i:xiM.i:i:\iKXT wnii clowixg it^atixial foil. 



In order to sliow the competition between -the rods and cones a 

 large strip of platinum foil is gradual^ heated by an electric current 

 from a dull reddish luminosity to a bright-red glow. Observed 



a Draper. Anier. .Jour. Sci. (!') 4. 1S47; I'liil. Mas. ('■') •"•'•. ^fay. 1.S47: Scieiit. 

 Vieinoii's. p. :y.i, London, ISTS. 



6 11. F. WVher, Berl. Alvud. Ber.. ISST, p. 4;»1 ; Wied. Ann. :',2. 2.')(i. ISST. 



