The Electiical Response of the Eye to Stimulation by Light 381 



Close to 1).^ there is placed a second lens L.^ which forms a sharp image 

 of L^ upon a tliird lens L.,. By this last lens there is formed a diminished 

 image of the spectrum wliich passes within the pupil of the eye and which 

 is denoted in the figure by 83. 



Tlu} light rays which pass out from S^ and are refracted by the lenses 

 and prism arrive weakened in S,. Through reHection from the refracting 

 planes and absorption in tlie refracting media, a part of the light is lost, 

 i^jut if we neglect this loss, then we determine as the basis of our construc- 

 tion that all the light rays of a definite wave-length which are collected 

 by the lens L^ from the slit must again be united in S3. 



Changes in the breadth of the slit S^ and in the horizontal dimension of 

 the rectangular diaphragm D.^ biing about some variations in the nature 

 of the light in use. If it is desired without changing the (juality to vary 

 the intensity of the radiation, then one must especially take into account 

 the height of the slit and diaphragm. We name the height of the slit and 

 of the two spectra respectively -s^, ti.^ and S3. The angle of aperture of the 

 cone of light falling upon lens L^ may be named <>^, that of the cone of 

 light passing from this lens fOj. The analogous angles of aperture for the 

 lens L3 may be named O3 and w^. 



In the space between the lenses, the prism, and the slit there is air with 

 the refractive index 1. On the other hand, we leave the possibility open 

 of filling the space between L3 and the eye with any medium, whose 

 refractive index may be termed n. 



According to known laws we nuiy write : — 



■i3=iilH_ (1) 



Sj n sin ^0)3 



If all the rays of light which are refracted by the lens L, p;iss tlirough 

 L,j, the numerical aperture of L3 must be equal to or larger than 7/ sin \u)^. 

 We therefore write 



n sin 1(03 p^Xa (2) 



where N3 is tlx- numerical uperture of the lens h.^. 



In the system of Icns.-s L„ (-mnol be dispensed with. If all parts are 

 exactly centred and if the requirem.'iit of formula (2) is satisfied, the light 

 rays wMiich are dii-ected uj^on the point of intersection of S.^ and the optical 

 axis, will all pass through the lens L.,. Here L^ does not function, but this 

 lens is required to bring about for all other points of S.^ what holds good 

 without the lens for the point of intersection above menti(ined. To attain 

 that object the, lens L.^ must form a focussed image of the second principal 

 plane of i^i upon the first principal plane of L3. 



From formuhe (1) and (2) it follows that 



*>1>1<S3 (3) 



In order to be certain that all the light which falls upon the eye can 

 always pass through the pupil, we have given to S3 a fixed value which is 



