96*8 The Diameter of the Pupil of the Eye in the Dark. 



Lei E be a position in which this is just possible, E beinor 

 nearer to the object-glass than is F the principal Eocut*. 

 Then if I) denotes the distance between the pinholes, d the 

 diameter of the apparent pupil, and v the focal distance AF, 

 the following equation is true. viz. : 



D 



d= . FE. 

 v 



In making this observation the eyelids must be held 

 against the open end of the tube B, not merely with a view 

 to exclude any external light but also to ensure a fixed 

 relative position of the eye and the tube. 



The tube B is now withdrawn, and a second tube of the 

 Bame calibre take- its place, of a length suitable lor dealing 

 with the second position E' where the pupil will have a 

 diameter equal to the distance between the beams of light. 

 A second adjustment is made resulting in the equation 



d= ]) . VVJ. 



r 



Adding the two equal ion-. 



. 1) EE' 



v 1 



Now, a- the two tubes employed have the same diameter, 

 the position of the eye relatively to the ends of the tubes 

 will be the same, or EE' will be equal to the difference 

 between the distances Erom the lens of the open ends of the 

 two tubes when in adjustment. 



Denoting therefore these distances by L and /respectively, 

 the diameter of the apparent pupil is given by the equation 



; 1) L-/ 



The following observations may be made. It is necessary 

 on approaching adjustment that the eye should be fixed 

 upon A, half way between P and P A . 



Let the diameter PP' be supposed horizontal, and suppose 

 the eye to be slightly nearer the object-glass than the 

 position E. Neither beam is seen if the eye is directed to 

 A, but it' the eyeball is turned to the right, the pupil is 

 shifted in the same direction, and P at once is seen. 



If the eye i> jnst on the side of E remote from the object- 

 glass, both beams are seen when the eye is directed to A ; 

 but a turn of the eyeball to the right puts P' out of view. 



