﻿80 Some Observations on Diffraction. 



77, 78. Perforated zinc. The former is developed to show 

 the rings in the spaces ; these are coloured, and the centre 

 may be white or not according to the size of the holes or 

 their distance from the screen. The latter figure is developed 

 to show the interior bands on the dark parts ; they naturally 

 give hexagonal systems. This may illustrate the markings 

 in Pleurosigma anqulatum, which are regarded as diffrac- 

 tion-bands made by the rows of white spots that form the 

 real structure of the diatom ; but, in truth, this case can be 

 realized without the aid of diffraction. Where there are 

 such rows of spots, there must be the lines of shadow between 

 them. The narrowest dark space marked over with dif- 

 fraction lines has two dark bars separated by a bright central 

 line. 



74. Arago's experiment. — The shadow of a threepenny-piece, 

 showing the bright centre. Here parallel sunlight was thrown 

 uncondensed on to the pinhole. The coin was about 18 feet 

 from this, held by a thin wire, and the screen of the camera 

 18 feet beyond, without any lens. The illumination was not 

 uniform, so that the exterior rings are partly seen : if a lens 

 is used, several rings may be seen around the bright centre, 

 but the complete shadow is too large for the field of view. 

 According to Verdet, Arago employed a circular disk 2 mm. 

 in diameter. 



75, 76. Arago's experiment. — Shot fixed on glass; 3 millim., 

 2 millim., 1 millim. diameter. These were taken with the 

 eyepiece and the ordinary arrangement. In theory there are 

 concentric rings over the whole shadow, but they are more 

 difficult to see than the bright centre. They appear in fig. 76, 

 and in a strong light may be produced with the larger shot. 

 It should be noticed in fig. 75 (PL IV.) and fig. 76 how large 

 a bright centre has been made in several places by specks of 

 dust. 



Both with the shot and the threepenny-piece, when they are 

 most accurately arranged, as evidenced by the regularity of 

 the rings, a faint dark spot may be seen at the centre of the 

 white spot, and in some positions of the disk relatively to 

 the eyepiece this opens out into a faint dark ring. At present, 

 I have not been able to ascribe with certainty a reason for 

 this secondary effect, but I think it arises because the source 

 of light is not a mathematical point. 



83 (PL IV.). Conical refraction; external. In conclusion, 

 1 venture to pass the strict limits of the subject by showing 

 the crowning triumph of the hypothesis of wave-propagation. 

 Light through 5 minute pinholes passing in a special direction 

 through a crystal of arragonite emerges in 5 cones of light : 



