i8o THE POPULAR SCIENCE MONTHLY. 



The first diagram exhibits the comparative size of a hole one sev- 

 enty-fifth of an inch in diameter, and the longest light-waves. If we 

 limit the aperture of the eye to this size, by holding a sheet of writing- 

 paper before it, with such a needle-hole pricked in it, and look through 

 the hole at a luminous point, such as a distant electric light, instead of 

 seeing it as a point of light too small to have a visible surface, as we 

 should expect, we will see instead quite a large disk of light sur- 

 rounded by one or two bright rings as illustrated in Diagram 5. 



ElAGKAM 5. 



This peculiar appearance is caused by the spreading of the light- 

 waves, after passing through the needle-hole, so that, although the 

 wave-fronts are spherical as they emerge from the lens of the eye, yet 

 at the distance of the retina they have spread out sidewise so much 

 that, instead of running to a point, they cover a surface large enough 

 to be distinctly perceived as a luminous disk. It can be proved mathe- 

 matically by the theory of undulations, that the diameter of this lumi- 

 nous disk, measured in seconds of arc as viewed from the center of any 

 lens, for light-waves, having a length of about -girortr of an inch (the 

 brightest and central part of the normal spectrum), will equal four and 

 a half divided by the number of inches in the clear aperture of the 

 lens, its size, however, increasing or diminishing a very little, accord- 

 ing as the light-waves are longer or shorter. 



Objects viewed through such a small hole appear very indistinct, 

 from the image of each point overlapping those of its neighbors. The 

 same defective vision would have resulted had the light-waves been 

 created less minute than they are, or of a size comparable to the diame- 

 ter of the pupil of the eye. 



It is also on account of the extreme minuteness of these waves 

 that light appears to travel in rays, and that opaque bodies throw 

 sharply defined shadows. 



Returning to a simple lens of considerable diameter, as shown in 

 Diagram 6, and still assuming it to have spheroidal surfaces so that the 

 emerging wave-fronts shall be spherical, and considering the light- 

 waves to be originated by a single vibrating molecule situated at an 

 infinite distance, we come to a peculiar phenomenon, also a result of 

 the excessive minuteness of the light-waves, and the consequent tend- 



