﻿Observations on Diffraction. 79 



61. Not quite a square, a bright square area with an in- 

 scribed dark circular area. 



65. A pentagon : here, as in several of the figures just 

 above, there is a bright spot at the centre. 



FresneFs Diffraction from Simple Objects. 



67. The inner part shows FresnePs interference-lines from 

 a bi-prism, with GrimaldPs fringes at the sides. 



68, 69. Narrow slits ; 68, with the broader bands, is from 

 the narrower slit. Both slits are so narrow that the exterior 

 bands, through interference of parts of the same wave from 

 one side, do not come in : the bands are given by waves from 

 the two edges. It may be remarked that in exterior bands 

 there is more indication of colour than in interior bands : 

 the neighbouring rays which form exterior bands can travel 

 almost parallel and make coarser bands with more chromatic 

 dispersion. In the case of light passing on to an aperture, 

 care is sometimes necessary to note which of the two effects 

 is predominant. 



70, 81. The eye of a bodkin, one to show interior bands, 

 the other to show the exterior bands : it is not easy to develop 

 both effects at once. The bands within the eye are not inter- 

 ference of waves from the tw r o sides, but the exterior bands 

 of each side : it happens that the breadth is such that a band 

 from one side is superposed on one from the other, so the 

 central line is dark. 



79. Four round holes of increasing size. This is a case of 

 exterior bands in the bright spaces ; the centre of the space 

 may be white or not according to the breadth of the hole : 

 also the rings and spots are brightly coloured. These holes 

 varied from ^ to 2 millim. diameter. If a hole be taken 

 ^ millim. diameter, the phenomenon changes and becomes 

 analogous to 68 and 69 : concentric spectra are formed in 

 the shadow, and there are no rings in the projection of the 

 aperture. 



71, 72 (PL III.). The points and eyes of ordinary needles : 

 the one with the broadest central line is the shadow of the 

 smallest needle which is made : here the interior and exterior 

 bands both appear. 



80. A quartz fibre about '0005 inch diameter. 



73. Wire gauze : each wire gives in shadow 6 dark bands: 

 the spaces give sometimes 2, sometimes 3 bands, showing an 

 inequality of distance. These bands are the exterior bands 

 from the sides of the wire, and give bright colours. The 

 general appearance to the eye is that of a Scotch tar tail. 



