386 Einthoven and Jolly 



The Ikrgest of tlie diaphragms used by us at Dg has, as mentioned in 

 Table II., a length of 275 mm. An image of it is formed in its full length 

 by the water-immersion lens D* in the pupil of the frog's eye. The image 

 so formed measures only 955 mm., while the objective is able to form 

 a circular image of 1*3 mm. diameter, although this image is very much 

 distorted at the margin. 



The diameter of the pupil of the eyes investigated by us has always 

 been greater than 0955 mm. 



The smallest diaphragm of each group in both tables is placed within 

 brackets, as it produces only on one condition the calculated weakening 

 of the light. The smallest diaphragm at Dj is only used when at the same 

 time the light is diminished 100 times at Dg, and the smallest diaphragms 

 of the three groups at Dg are only used when at the same time the light 

 is 10 times weakened at D^. 



If these conditions are not satisBed the diminution of intensity becomes 

 greater than that calculated on account of the diffraction of the light. It 

 can easily be shown that when the conditions are satisfied, the influence 

 of the diffraction upon the image formation at all three places S^, S3, and L, 

 may be neglected. 



5. The Spectrum. 



The collimator consisting of the slit S^ the lens L^ and the tube con- 

 necting these two parts, is fixed upon a horizontal plank which may be 

 rotated around a vertical axis. The continuation of this axis runs in 

 the plane which bisects the refracting angle of the prism P, and in order 

 to be able always to obtain the minimum of deviation the prism may also 

 be rotated separately around this axis. 



The prism, which has walls of mirror glass, is filled with carbon 

 disulphide and is 115 mm. in height. Its base is an equilateral triangle 

 whose side measures 105 mm. 



The lamp is placed on the same horizontal plank as the collimator, and 

 always therefore rotates together with this. 



The whole is placed upon tables which are nailed to the floor, so that 

 the axis on which the lamp, the collimator, and the prism rotate holds an 

 unchangeable position with regard to the system of lenses and the eye. 



There are, as already mentioned, for illuminating by the three parts of 

 the spectrum, three rectangular diaphragms in use which can successively 

 be placed at Dg. In order to be able to judge if the spectrum forms here 

 a sharp image, and if in fact only light of the desired wave-length passes 

 through the diaphragm, the pasteboard tube which lies between the prism 

 and D2 for the purpose of excluding the daylight, is taken away and the 

 spectrum is directly viewed. The self-regulating lamp \s replaced by an 

 arc lamp with hand regulation which can easily be done without displacing 

 in the slightest the collimator or other parts of the installation. Between 

 the carbons of the hand-regulated lamp, a salt, either of sodium or of 



