1180 
a measurement by PurrricH; D, = angle of minimum deviation): 
} n D, À n D, 
B 0.687 1.3071(P) 21°37 «x OA9L 1.3136 22°7' 
C 0.656 1.3079(P) 21%41'’ F 0486 1.3140P) 22°8' 
D 0.589 1.3098(P) 21°50 y 0.449 1.3157 22°16! 
E 0.527 1.3121(P) 22°0' G 0.431 1.3168 22°21’ 
w and y are two special wave-lengths which are of importance for 
the deduction of the colour-effect following further on. 
For these eight colours and for angles of incidence between about 
41° (corresponding to the angle of minimum deviation, which differs 
for the various colours) and 62° the expression (1) was computed. 
In this set angles of deviation up to 25° are included. Larger angles 
of incidence were found to be unnecessary; and, moreover, the loss 
of light by reflection probably begins to exercise its influence in 
this region. | 
The results are contained in the accompanying table. A special 
calculation for the line / which differs very little from we was not 
carried out. Fig. 2 gives the dependence of the intensity on the 
deviation. 
Fig. 2. 
By means of these curves the intensity was determined in direc- 
tions from 21°30' upwards ascending by 15’. It is necessary in this 
computation to bring into account the finite extension of the sun. 
The same approximation was applied as used by PerNrer in the 
case of the rainbow; in fact Prernrer’s method of computing the 
colours was followed throughout’): the curves are shifted in three 
steps of 5’ both to the left and to the right and read each time. 
The figures thus obtained, corresponding to seven points of the sun 
at intervals of 5, are added up. The practical execution of the 
method comes to reading the curves from 5’ to 5’ and each time 
') Pernter, l.c. pag. 529 sqq. 
