( J49 ) 



cones, ill llic hiKer tlic fine siiu'lliiiu-liairs wliicli hodi lunii |»;ii'l of 

 llio Jerniiiial ikmitoh. Wc will considcM' llic stiiniil.ilioii (ff lliese senses 

 a litUe more closelv. 



A. Organ of sight. 



If one looks llii'oiigii an ai'lilicial oi'ilice of '2 mm. at a small 

 Hefner lamp a( a dislance of (> meli-es, a dclinih^ ainoiml of (he 

 lio-jit emitted hv the lamp ^vill enl(M- the eve. 'J'his enei'U'v is eon- 

 eentrated on a small eirenmscript Held of the retina, where cones and 

 rods lie readv to r(M'ei\(' the light-stinudiis. When llu' room is maile 

 dark and the light is made f'eebkM- hv a system of more or less 

 crossed Nichols, the much less sensitive cones will at last cease to 

 he active and the feeble glimmei- that i-emains, w ill Ik» |)ercei\ed l»v 

 means of the rods oidv. For this the \ isiial axis w ill Iia\e to dexiate 

 a little, as in the fovea centralis itself no rods occur, so that the 

 point-shaped feeble little star seems to be displaced a little upward, 

 at any rate for my eye. 



It is clear that with such an arrangement of the experimejd those 

 i-ays oidy will be eliective that are absorbed by the purple of the 

 I'ods. According to A. K()mo ^) these are the rays the wa\e-length 

 of which ranges from ()()() to 420 micra. From K. ANCiSTuöM's recent 

 determinations "-) we know the energy of this [)art of the spectrum 

 for the llefnei- lamp. It is '2M\ XlO"'^ gram-calories per second 

 and j)er sipiare centim. at a distance of one meli-e. At a distance 

 of (J metres this becomes 0.03 ergs per second and per sipiare c(Mitim. 



In this experiment we sup|)osed the Hefner lamp to be looked at 

 through a system of moi-e oi' less ci'Osse<l Nichols, leaxing oid\' a 

 feeble glimmer. IJesides this \ve will also iuscrt an instantaneous 

 shutt(M- the time of exposition of which has been adjusted so as lo 

 give the most favourable results for feeble \isual impi-essioiis. From 

 a series of experiments by Messi's. (Jki.ins and Noyons, the results of 

 which will be commuincated later by these genlhMuen llieinsel\es, 1 

 knew that a time of exposition of the oimIcm- of a iuilli-s(M'oud is most 

 suitable. An insti-umeid formerly used by Dr. Lvvn and described in 

 his doctor-dissertation ■''), \vith slits mo\ inu in o|t|»osite <lii-ections, gave 

 expositions of about 0.(1 milli-second. i-epcalcd cxcry 0.0() second. 



Uy these two means in addition to the nai-row ness of oriticc, hence 

 in all by three circumstances: 



1) A. KöNiG. Stzlier. d. Bei liner Akadoniio 1S94 p, r)85. 



~) K. AiVdSTRÖM, the Physical P.cvifw, Vol. XVlll p. 'M)2, lOO'.}. 



^) H. A. Laan. Onderz. Physiol. Lab. Utreclilsclic Hoogeschool. (5) III p. 182. 



