1185 
a very large number of rectangular apertures, slits, spread at 
random. Diffraction fringes will be formed parallel to the refracting 
edges of the crystals and a rotation of the plane about the line 
eye-sun produces diffraction circles round. the sun. 
The places of the diffraction minima are given by the equation 
À 
sn Ö == M=, 
a 
where @ is the angle of diffraction for the minimum, m the order, 
À the wave-length and « the width of the slit. 
The first maximum for each colour is formed in its minimum 
deviation. Towards the outside of the ring each direction represents 
a maximum for each colour accompanied on both sides by 
diffraction fringes. The resulting colour cannot be anything there 
but white and the only colours which can be seen will have to be 
looked for on either side of the first maximum. 
In each direction two beams AB and HK (fig. 1) of different 
width cooperate. Considering that these beams approach each other 
in width the nearer they are to the minimum-deviation and that in- 
its neighbourhood the deviation of the rays changes very slowly 
with the angle of incidence, it follows that the diffraction lines very 
nearly cover each other in the neighbourhood of the minimum- 
deviation and hence that the colours must be more prominent there 
than elsewhere. In order to elucidate this effect | have computed 
the relative widths of the slits A5 and HK (equation 1) on -both 
sides of the minimum for angles of incidence between 37° and 50° 
| max.— min. 
A Io a as D 
ay Ay 
40°55) 40°55/ 0.4363 | 0.4363 | 100° | 100’ 21°50/ 
42 0 39 51 4228 | 4306 | 103 101 51 
430 | 3853 4104 4250 | 106 103 53 
440 | 3755 3980 | 4190 | 110 | 104 55 
450 | 36 50 3856 4130 | 113 | 106 59 
460 | 36 3 3734 | eer 22 3 
drol ag 6 3612 121 8 
48 0 34 14 3490 | 125 14 
49 0 33 22 3370 | Hsiao ee: 
50 0 32 30 3250 ok ames | 30 
