﻿Observations on Diffraction. 73 



to act as a rectangular grating. If the eyelids are adjusted 

 to the lamplight, 8 or 10 horizontal exterior bands will easily 

 be produced. Most plate-glass windows have become gratings 

 through the scratching by repeated cleaning. 



To return to the main experiment. Stronger effects are 

 produced, of course, when sunlight is condensed on to the 

 pinhole, and in this case the position of the convex lens 

 for giving parallel rays on emergence, or for giving the plain 

 image of the object without diffraction, is not exactly its focal 

 distance from the object, for the divergence of the incident 

 rays has some influence ; but there always is such a position. 

 Moreover, the elementary experiment cannot be made with 

 the Huygens eyepiece which is now used, for it has not an 

 external focus. Fig. 8 shows the result of placing the object 

 close up to the eyepiece ; the beginnings of diffraction may 

 be seen. Fig. 10 (PI. I.) shows further developments when 

 the distance is increased, and figs. 9, 11, and 12, 13 (PL II.) 

 give the best effects I have at present been able to produce. 

 In these and in most of the other figures a magnify in g-glass 

 is necessary to reveal all the fine detail made by the crossing 

 waves in the shadows. It is clear that if the object is capable 

 of giving spectra passing out with much obliquity, they can 

 never come into the field of the eyepiece by this method of 

 observation ; and the eyepiece can never focus them, as it 

 does in the Fraunhofer method, in which case the field-glass 

 brings them to the focus of the eyepiece. 



It may be said that the Fraunhofer diffraction gives nume- 

 rous images of the source of light set in radiating forms which 

 depend on the shape of the object, while that of Fresnel com- 

 monly gives the main shape of the object embellished with 

 fine detail in the spaces between its various parts, and some- 

 times curiously inverted in its leading features. 



A simpler form of this arrangement demonstrates a leading 

 idea in wave theory : parallel sunlight falls upon a convex 

 lens, while the diffracting object is between this and the 

 screen of the camera, which has no lens. Fair diffraction- 

 effects are made when the object lies in the converging rays, 

 and they are better when it is moved to the divergent pencil, 

 but when it is at the focus the plain original figure is repro- 

 duced. At the focus the several rays that started from one 

 point of the sun have again all reunited ; so that in passing 

 the object, although there are many separate rays, there are 

 no two rays which came from the same original point ; and 

 it is only such as these which can produce interference. 

 Here may be seen, in a certain sense, an object and real 

 image on the same side of a convex lens. 



