with Special Reference to the Vision of Insects. 319 



of a star) consists of a small round central patch called the 

 spurious disk, surrounded by coloured rings which very 

 rapidly fall off in brightness. This phenomenon is due to 

 the interference of the light coming from the two halves of 

 the object-lens, and is susceptible of mathematical treatment. 

 It thus appears that the angular radius of the first dark ring, 

 estimated from the middle of the object-lens, is 



= (1-22) J, 



where X is the wave-length of the light, and A the aperture, 

 i. e. diameter, of the object-lens. This furnishes a boundary 

 within which the central spurious disk lies, and up to which 

 its faintest outlying portion barely extends. It also fixes the 

 minimum visibile with that aperture, since two points would 

 have begun to be blurred into one another if so close that the 

 middle of the spurious disk of each lay on the first dark ring 

 of the other. Let us then put into this formula, = 1' = '00029 

 in circular measure (this is the limit already fixed by the rods 

 and cones), andX=*6 of a micron (which is the wave-length 

 of yellow light). We thus find 



•00029= (1/22) j 



whence A =2524 microns, which is very nearly -fe of an 

 inch. This, then, is the diameter of the pupil of the eye 

 when of such size as to put the same limit on the visibility of 

 small objects as the rods and cones do. Now, this is about 

 the size to which the pupil of the eye shrinks when we 

 scrutinize well-illuminated objects, and is the smallest to which 

 it can be allowed to shrink without interfering with the vision 

 of minute detail, by placing a further restriction beyond that 

 imposed by the layer of rods and cones *. 



Again, the eye viewed as an optical instrument is far from 

 perfect. Its chromatic defect may be detected by placing 

 the finger horizontally in front of the eye, and looking just 

 over it at the bar of a window. In this way the window-bar 

 is viewed through the upper half of the pupil, and is then seen 



* It might be thought that with the more dilated pupil which we have 

 in faint light, we could see more detail. But the reverse is the case ; 

 for instance, the two small double stars e : and e 2 Lyras are more than 3' 

 asunder, and yet, in consequence of their faintness, are nearly at the limit 

 of what a very good eye can see distinctly as two objects. To eyes that 

 are fairly good they appear as one object elongated, while persons may 

 have tolerably good sight and not even see the elongation, 



