382 Einthoven and Jolly 



less than the pupil diameter of any of the frogs' eyes which we have 

 investigated. The question above stated as to the maximum of light 

 which can be radiated from the crater upon the pupil now reduces itself 

 to the question as to the most favourable values for s^, sin ^o^, and N3. 



The basis of our constiuction was, as we mentioned, that all the light 

 which falls upon L^ shall enter into the pupil. If we were to leave N3 

 out of account, then a maximum of light would fall upon the pupil when 

 a maximum of light radiates upon Lj. For that purpose one must give 

 a maximum value both to s^ and Oj ; in other words, the slit must be large, 

 and at the same time the collimator lens L^ must have a large diameter 

 and a short focal distance. 



But proportionally to the increase of s^ sin |0j, Ng must also be in- 

 creased, and in this we soon practically reach an unsurmountable limit. 

 It is among microscopic objectives that one can find lenses with the 

 greatest numerical aperture. We selected from Carl Zeiss' catalogue the 

 water-immersion lens D* as the most suitable lens for our purpose. The 

 numerical aperture of the lens is 0-75, while the free object distance 

 amounts to TS mm. The lenses with greater aperture which are 

 mentioned in the catalogue have all a much smaller object distance which 

 renders them unsuitable for our purpose of forming the light image in the 

 pupil plane of the intact frog's eyeball. 



Formula (3) can be written in the form 



«>5i£i<N (4) 



''a — 

 In our experiments Sj = 19 mm., sin JOj = 118 mm., and S3 about 033 mm. 

 From this it follows that 



^jA»J^i = 0-68. 

 S3 



N3 having as mentioned the value of 075, the requirements of formulae 

 (3) and (4) are satisfied, and by an appropriate arrangement we can ensure 

 that in fact all the light falling upon the collimator lens L^ enters through 

 the pupil into the eye. 



The slit Sj is only a few millimetres distant from the crater. We 

 could, of course, easily have placed a higher slit behind a larger crater, and 

 much stronger collimator lenses with greater diameter are available, but 

 by making use of such means we could not in any case, as we mentioned 

 above, have increased the illumination of the image in the pupil. 



The intensity of illumination which we obtained by means of our con- 

 struction with a lamp of 20 amp. and a simple collimator lens was already 

 very great. If one imagines in place of the slit of 19 mm. in height a 

 square diaphragm with side Sj = l-9 mm., and if the rays are not dispersed 

 by a prism, so that in place of a spectrum of height 033 mm. in S3 a 

 square white spot of light is formed of side S3 = 033 mm., then the 

 illumination of this spot can be expressed in metre-candles or Lux. 



