384 Einthoven and Jolly 



metre-candles.' Our image in the pupil of the frog's eye is therefore 

 rather more than 400 times more strongly illuminated than this. 



As fig. 1 is only a diagram it does not show the dimensions of the 

 system of lenses which we have used. We therefore give here some of 

 the actual dimensions : — 



Height of slit . .1-9 mm. 



Breadth 



Distance of S^ from the anterior surface of Lj 



Diameter of Lj 



Distance of the posterior surface of Lj from 0-2 



Distance of D2 from L3 about 



11 



1650 



39-2 



825-0 



1000 



4. Weakening the Light. 



Our original plan was to weaken the light by means of smoked glasses 

 in order to leave unchanged the form, magnitude, and colour of the retinal 

 image, and to vary solely the intensity of the light. But it soon became 

 apparent that even the best of the so-called neutral glasses, if they absorb 

 a great part of the light, do not allow all colours to pass through equally. 

 Before they could be used it was therefore necessary to measure the trans- 

 mitting power of these glasses for each colour separately. 



As we had not an opportunity of carrying out these measurements in 

 a sufficiently accurate way, we have contented ourselves with the use of 

 diaphragms. In the first place the opening of lens L^ can be diminished 

 by diaphragm Dj, and in the second place the opeiung of lens Lj by 

 diaphragm Dg. 



The first diaphragm, which varies the illumination intensity of the image 

 S3 in the pupil, leaves, it is true, the form and magnitude of this image un- 

 changed, but diminishes the spot of light on the retina as it considerably 

 diminishes the size of the diffusion circles which contribute not a little to 

 the formation of the retinal spot. 



The .second diaphragm Dg diminishes the image S3 in the pupil, 

 whereby a further diminution of the retinal image is brought about. 

 Moreover, D^ intercepts the rays of light at the margin of S2. which have 

 another wave-length than the rays at the centre, but the colour of the 

 retinal image is thereb}' very little altered. 



By measuremejits which have been made to determine the sensitiveness 

 of the human eye to very weak light, it is found that if the spot of light 

 on the retina does not exceed certain limits, the illumination of the spot 

 that is necessary to give rise to a sensation is inversely proportional £0 its 

 area. In these circumstances the quantity of energy required for a sensa- 

 tion is therefore independent of the size of the retinal area illuminated.* 



If the spot of light is not small enough the rule above mentioned does 



' Arrhenius, Lehrlnich der kosmischen Phyeik., Leipzig, p. 93, 1903. 

 ^ Piper and Asher ; cf. v. Kries, Zeitschr. f. Psychol, u. Physiol, d. Sinnesorg., 2 Abt., 

 Bd. 41, 1907, pp. 376 and 377; also cf. Henius, Zentralbl. f. Physiol., Bd. 22, p. 229, 1908. 



