﻿Observations on Diffraction. 71 



Although more often named after Fraunhofer, as is due to the 

 first great worker in this direction, high tribute is always paid 

 to the more perfect consummation of theory and experiment 

 which was published by Schwerd of Spires in 1835. 



A telescope is focussed to a star or a distant point of light, 

 and various limiting apertures are placed on the outer side of 

 the object-glass. Consider, first, the case of two or three narrow 

 parallel lines of light : they constitute an elementary grating, 

 and light from the star passing from them to the object-glass 

 will come as if from several stars lying on either side of the 

 real star, all being spectral images except the central one. The 

 object-glass focusses these at the usual place, then the eyepiece 

 magnifies them and transmits parallel rays of light ; so there 

 is a broken line of light across the field, consisting of the 

 plain image at the centre and the spectra of the successive 

 orders on either side. More complex limiting apertures give 

 these radial groups of images of the source of light, of which 

 the number and direction are regulated by the general form of 

 the aperture : thus a triangle will give three lines across the 

 field or a six-rayed star, a square will give a four-rayed star; 

 a circle concentric with the object-glass, however, will give 

 concentric circular spectra. 



The objects whose effects have been photographed consist 

 of combinations of thin circular lines of light on a dark glass 

 plate | inch diameter ; this fits into a cap at the end of a 

 telescope, and each figure in turn is brought to a small 

 opening in front of the centre of the object-glass : this is 

 known as Bridge's apparatus, constructed some years back 

 for this form of diffraction. Fig. 8 almost represents these 

 figures ; it has slight traces of diffraction of the other kind. 

 Figs. 1-7 (figs. 2, 3, 4 on PI. I.) represent the Fraunhofer 

 diffraction of the seven more complex figures : the simplest 

 figure, which consists of two concentric circles, gives con- 

 centric spectra by both methods of observation. 



2nd. The Diffraction in Sliadovis : Fresnel's Diffraction. 



This is described by Lord Rayleigh as the Diffraction when 

 the source of light is not in focus, or in Jamin's Cours de 

 Physique as Diffraction from a Spherical Wave. Light is 

 condensed on a minute pinhole in a thin metal plate ; about 

 a foot from this the diffracting object is placed, and at a 

 similar distance beyond a microscope eyepiece receives the 

 shadows : they may be observed with the eye or projected on 

 the screen of a camera for photography. The rays emerge 

 from the eyepiece parallel, and give an image on the screen 

 which varies in size but not in quality as the screen is moved ; 



