contradiction with those of others. This will all be shown further on. 
In this paper an attempt will be made by a modification of the 
theory in which diffraction will be taken into account to explain 
the colour-phenomena as observed. 
We shall begin by tabulating a number of the various colours 
that have been recorded, chiefly taken from the publication of the 
“Koninklijk Nederlandsch Meteorologisch Instituut”: “Thunder storms, 
Optical Phenomena ete. in Holland according to voluntary observ- 
ations 19011914"). This will be followed by a discussion of 
the simple refraction-theory for ice-erystals with a refracting angle 
of 60° in order to arrive at the colours which might occur in the 
ordinary circle. A  diffraction-theory will then be developed and 
finally the colours will be deduced for a specially well developed 
halo of 22°, which will appear to agree very well with the observations. 
I. Survey of some of the colours observed in halos. 
I shall confine myself to those records, in which colours are 
mentioned by name. Lyrical rhapsodies like: ‘brilliantly, very 
intensely, strongly, magniticéntly coloured”, especially” numerous 
for the parhelia, cannot be utilized. In the fourteen volumes of 
“Thunderstorms ete.” which L consulted the colours of the parhelia 
are named in only five cases! 
With regard to the cireumzenithie are Bresson expresses himself 
as follows ’): 
Les couleurs sont souvent remarquablement pures: on distingue 
en bas le rouge, puis le jaune en passant par lorangé, puis le vert, 
puis mais pas toujours le bleu et le violet. Cette derniere couleur 
est fréquemment absente, mais il n'est pas rare, quelle soit visible 
tres nettement. Fait important a noter: le violet, quand il existe, 
est trés pur, il n’est pas surmonté ou mele de blanc, comme dans 
les ares tangents au halo de 22°. La coloration offre une intensité 
des plus variables: faible parfois an point d'être a peine perceptible, 
elle est d'autres fois aussi éclatante que celle du plus bel arc-en-ciel. 
These references, incomplete though they are, are sufficient 
to show the great variety of colouring in the halo-phenomena. 
Especially important from this point of view are those cases in which 
different observers mention the same colours. 
1) Onweders, optische verschijnselen, enz. in Nederland naar vrijwillige waar- 
nemingen 1901—1914. (Cited in the following table as: “Onw,”). 
2) L, Besson, Sur la Theorie des Halo’s, Paris 1909, p. 53, 
