OBSERVATIONS ON VISION. '251 



coiidiU'liou (coiitacl Ihcory). \\'li!l(' the t'liiid ic i of these coiiliicl 

 stations has been coifipared to that of an electrical interniptor. I am 

 inclined to think lliat thei-e is here iiisei'led in Ih'' coiidnctini;' path a 

 potential capacity which increases the sel I'-indnct ion (\i\v to the propa- 

 li'ation of the eh'cti'o-niaaiK't i<' liii'ht \\a\('s, in re\'ei'se of the })rocess 

 of Piipin for te]e|)honini>' o\'er i>reat distances i)y means of cables. 



('. Exi'KIM AIKN rs ox \^ISI(»X IX 1 )UI(; I ITX KSS AM) ( )r.S( l Kl'l ^ . 



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

 coni])etition. t!ie fodowinii' experiments :ii'e inti'odnced: 



r. i)K\ KLoi'MKxr OK iriK sim:<"1'i;i M r.v oiiAoi al iikaiixc; ok ax incax- 



DKSCKXr I. A Ml'. 



An incandescent lamp with larac lilamenl is placed in a snitalde 

 box. and means are j)ro\ ided for iiradiiaJly varyiiiii' I he healini:- cur- 

 rent between the linuts cori'espondinu" to (]ull-red olow and incandes- 

 cence. The lilanieiit serves as the soui'ce of lieht and a lai'<i(' spec- 

 ti'um is thrown on a white screen by means of a Row land concaxc 

 ii'rating". After the eye has become accustomed to comj)lele dark- 

 ness the current is increased until the lirst aj)[)earance of liojit is 

 recognized on the screen. Idiis first reco<»;nizal)h' light is colorless, 

 and makes the im])ressi<)n of "rod-white" luminosity. As the cui-- 

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

 pears to stretch farthei- toward both ends of tlie spectrum, and sensa- 

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

 coloi' begins to appeal' the interesting obser\at ion is made that the 

 j^osition of apparent maximum intensity, lying lirst in the blue green, 

 is (lisi)laced more and more toward the yellow green of the spectrum. 

 This dis[)lacement may be conlirined by marking the brightest spot 

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

 white luminosity remains. 'i1ie spot where this is seen brightest is 

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



From this exj)eriment it follows that the rods first percei\-e light, 

 and that their greatest seiisiti\'eness is for the blue-green reiiion of 

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

 experiment is founded on the researches of Draper" and 11. V. Weber.'' 



II. KXi'KKi MKX r wrni (irowixc; rrATixiM koii,. 



In order to show the competition between the rods and cones a 

 large strip of i)latinuin foil is gradually heated by an electric current 

 from a dull reddish luminosity to a brighl-red glow. ( )bser\ed 



ar)rai»er. Amor. .Tour. Sei. (2)4, 1S47; riiil. Ma^'. (::) I'M .M.-i.v. 1S}7: Sciciil. 

 i\feiuoirs, p. ;;:!, I.ondtui, 1S7S. 



Mi. l\ Welier, P.erl. Alcad. tier.. 1SS7, p. I'.lt ; Wied. Ann. If-', _'.".<;, 1SS7. 



