The Electrical Response of the Eye to Stimulation by Light 387 



lithium, is then placed and the light of the arc exhibits a sharp spectrum 

 of lines in Dg. 



The dimensions of the rectangular diaphragms placed here, which are 

 given in Table 1., are so chosen that the spectrum in its vertical dimension 

 radiates exactly through the opening of the diaphragm, while each 

 rectangle is so long that two previously determined easily recognisable 

 spectral lines fall upon its lateral margins. 



For illumination with red the lithium line X = 0'67 and the sodium 

 line. X = 059 are thrown upon the left .and right margins respectively of 

 the rectangle Dgr- For illumination with green the sodium line and the 

 lithium line X = 0497 are thrown upon the left and right margins of the 

 rectangle D^, and for illumination with blue the lithium lines X = 0'497 

 and X = 0"460 are thrown upon the left and right margins of Dab- 

 When the spectrum is focussed upon Dg, in order to illuminate with 

 one of these three parts, the exact position of the collimator and the prism, 

 which can be easily controlled, is read from a scale. After both of these 

 have been firmly screwed in their places, the hand-regulated lamp is 

 replaced by the self -regulating lamp of Siemens and Schuckert. 



6. The Movable Screens. 



We must now describe the arrangement which enabled us to cause the 

 light to enter the eye at the desired moment and to radiate during any 

 desired time. This arrangement, which agrees in principle with that used 

 by de Haas,^ is placed between the prism and Dg and consists of two parts. 

 The first part is composed of a system of two black, equal-sized, vertically 

 placed discs which may be rotated upon a common axis at right angles to 

 their centre. This axis is parallel to the course of light rays VS^. 



The discs are pressed one against the other and cover one another 

 completely, but a portion is cut out from the circumference of each, and 

 by rotating one disc oyer the other, an orifice is left of any desired breadth 

 through which the light can pass to the eye. 



With the aid of an electromotor the pair of discs can be rotated as 

 one with exactly determinable velocity. The speed joi rotation and the 

 breadth of the opening at the circumference determine tpgether the 

 duration of illumination. 



In order to ensure that the light does not pass with every rotation, the 

 second part of the apparatus is constructed. In this second part a small 

 screen, formed like a semicircular disc, is present. By the aid of a strong 

 spring the disc is rotated when a catch is withdrawn, until after half a 

 rotation it is stopped by a second catch. In one position of tho screen 

 the light rays may pass, in the other they are intercepted. 



The catches art; brought into action by an electromagnet which is 

 ed by a battery of accumulators. The circuit wiiich carries the current 



' Loc. cit. 

 VOL. I., NO. 4. — 1908. 26 



