﻿72 Mr. W. B. Croft : Some 



infinite changes, however, are made by variations in the 

 relative distances of the source of light, object, and lens or 

 eyepiece. 



In order to consider a fundamental principle of importance, 

 take at first more simple apparatus, a convex lens instead of 

 the Huygens eyepiece, and let the object, which is the glass 

 plate with circle combinations, be illuminated with parallel 

 uncondensed sunlight. If the lens is placed at its focal 

 distance from the object, rays emerge parallel and make 

 simple images of circles without any diffraction developments ; 

 they are always in focus wherever the screen is placed, and 

 are naturally greater when it is more distant. If the lens 

 is slightly moved from this position in either direction, the 

 emerging rays cross one another and produce interference 

 developments. The figures made when the lens is moved 

 towards the object are inferior in definiteness. 



A wave impinging on an edge gives rise to the secondary 

 waves of Huygens, and from each new centre waves spread 

 out in all directions, although the incident light may have a 

 plane front or be a parallel pencil ; but a pencil, after emerging 

 parallel from a lens, cannot be considered to have rays striking 

 out obliquely so as to interfere. From this may be imagined 

 the formation of diffraction-rings in a telescope directed to a 

 star when the eyepiece is moved so that the image made by 

 the field-glass is out of its focus ; the image is nearly a point 

 of light ; while the rays emerged from the eyepiece parallel 

 they would not interfere, but directly they cross one another 

 on emergence they give rise to systems of concentric rings. 

 This phenomenon may be well seen by looking through the 

 eyelashes towards a distant lamp while a fine rain is falling : 

 minute spheres of water fall upon the eyelashes ; these short- 

 focus lenses make images of the lamp in front of the eye, and 

 the lens of the eye cannot focus or turn parallel the rays from 

 these points ; for a moment a number of fine concentric rings 

 may be seen, which are constantly evanescent and constantly 

 renewed by fresh-falling specks of water. 



There is another phenomenon sometimes revealed when a 

 divergent pencil from a short-focus lens or spherical reflecting 

 surface falls upon the eye in such a way that the eye cannot 

 focus it : a small illuminated disk is seen with fine ramified 

 lines from the centre ; in this the eye sees its own blood- 

 vessels : this is easily seen by holding a pinhole near to the eye. 



Abundant examples of diffraction may be observed without 

 any apparatus : a distant lamp viewed through the eyelashes 

 on a dry evening appears with many spectral images made by 

 this simple grating : an umbrella can be seen in the same way 



