Physics. — “On the. difiraction of the lijht in the formation of 
halos’. By Dr. S. W. Visser. (Communicated by Prof. J. P. 
KUENEN). 
(Communicated in the meeting of March 31, 1917.) 
I. Introduction. 
The halos which originate by refraction are often seen distinctly 
coloured. This fact is best known as regards the circumzenithic arc, 
the parbelia and the tangential ares, but the large circle is also 
often coloured and in the ordinary circle of 22° too, distinct 
colours occur. 
In this paper the last circle will be principally dealt with. 
According to the common refraction-theory of halos, as developed 
by PeRrNTER*) amongst others in his well-known work, the red on 
the inside of the circle should be distinctly visible, whereas the 
green and blue would already be very pale. The observations on 
the other hand show, that the colours are often practically invisible, 
but that sometimes they appear with great brightness, and the same 
holds for the other halo-phenomena mentioned above. These bright 
colours are explained by Perrrer as regards the parhelia and the 
tangential ares by the presence of a large number’ of ice-crystals, 
whereby the intensity of the light would be inereased and the colour , 
become visible’). But if there is nothing but white light or nearly 
white light, an increase of intensity cannot produce anything but 
more white: the distribution of colour cannot undergo any change 
by a greater intensity of light. 
A remark by Perrnter himself shows, how little the colours fit 
into the usual theory ®): 
“Tt should be mentioned, that in a description of the phenomenon, 
as observed on September 4'' 1900 at Aix la Chapelle by SIEBERG 
I find the following statement as to the colours of the smaller circle: 
“In addition it was distinctly coloured red, yellow, green and blue 
from inside to outside”. As this observation would contradict all 
others, if it referred to the complete ring, I assume, that the colours 
were observed, where the parhelia were situated on the ring, in 
which these colours have also been observed by others”. 
But the colours of the parhelia are not in accordance with the 
theory either and moreover SIEBERG’s observation is „ot at all in 
'!) J. M. Pernrer and F. M. Exner, Meteorologische Optik, Wien 1910 pag. 319. 
2) Pernter. l.c. p. 318, 320, 321. 
3) PeRNTER, lc. p. 228. 
