OMSKRVATIONS ON VISION. ^Tj.'i 



(The writer coiiiuH'ts wilh this plu'noiiiciioii, which was first ol)- 

 served by Von Bozofd uiiiJ later more exactly iii\'<'stii»ate(l by Heriuii 

 aiul Hillebrand. the origin of ^he theories of Kchiio- and ^^on Kries.) 



VI. TIIK PIKKIX.TK rilENOMENOX WITH I.AK<;F. AND SMALI, I'IKI.DS. 



(a) Lavije ft'lds. — A projection ap})aratiis with two hir<2:e Nicol 

 prisms is suitable for the demonstration of the Purkinje phenomenon. 

 For this purpose tlie slit is replaced by a round opening, and an 

 enlarged image is thrown upon the various colored fields to be com- 

 pared. These are preferably of colored paper, from which four 

 arcs are combined to form a large field. Reel and blue-green are 

 suitable colors and are to be so chosen that with the strongest avail- 

 al)le lighting the red is perceived to be more bright than the blue- 

 green. In case suitable })ai)ers are not available, the same efi'ect can 

 be reached by interposing in the l)eam suitably colored glasses or 

 stained gelatin films. If the brightness of the light is gradually 

 diminished, the Idue-green becomes at length brighter than the red, 

 and finally the red disa})pears, while the blue-green changes to the 

 colorless " rod-white."' 



(J)) Ohserrafioii of snidU pchls. — The large field just described 

 is covered with black i)aper or velvet, except in two snuiU spots, where 

 the colored paper may be seen. Now, however much the intensity 

 of the light is diminisluMl. the red always appears bi'ighter than the 

 l>lue-green, provided both spots are observed directly so that their 

 images fall on the fovea centralis or macula lutea. and not toward the 

 ])eriphery of the retina where both rods and cones are j)resent. 



(r) Alterudte ohscrrdfion of the hnuje <iii<I .s//i(fl/ fichh. — In order 

 to make the competition of the rods and cones still more striking, the 

 two fields may be obs(U'\'ed alternately. Having reduced the inten- 

 sity of the light on the small field, all the wdiile recognizing the red 

 to be brighter than the blue-green, the screen is remo\'ed and the eye 

 sees, again, suddenly, the large field. The blue-green [)arts now 

 a])pear colorless, but of a magical l)rightness, while the 1'(m1 [)arts 

 become almost black. 



The Purkinje ])hen()menon may l>e observed in ordinary conditions, 

 iis when a picture gallery is \isited on a very cloudy day or in the 

 twilight, for then all the red tints apipear dark and obscui-e, and all 

 the blue ones coloi-iess and whitish." 



Ilelmlioltz also mentions that of all objects the blue sky appeared to 

 retain color longest at twilight, and I believe I am right in assuming 

 that the moonlit landscape is " i-od-white," for at least the silver lus- 

 ter of moonlieht is verv similar to the ffhostlv irrav-white of the rods. 



"II. V. Helniliolt/. : Il.-mdbiicli dcr I'liysiol. Oi)tik. 2 eil. ]>. 4l'!». ( Lt^opold 

 Vos.s, Leipzig, 1S9(). ) 



