1192 
September 19 1905, the same afternoon as Hissink’s second obser- 
vation. The application of the calculation to the upper tangential 
curve is allowable, at least in the neighbourhood of the point of 
contact, as the refraction takes place at that point in exactly the 
same manner as in the ordinary circle; the colours may thus be 
looked upon as belonging to the latter. 
The agreement with the colours as observed at Nymegen and Zutfen 
is nearly complete: only the second violet is absent in the caleulated 
set. The observer at Nymegen reports: red, yellow, blue (wide), 
violet (narrow). The calculation for 22°10' and 22°5' gives green- 
blue (colour-numbers 13.1 and 13.2) very near blue. Green is absent 
and blue has a width of 40’. The violet is nearly exhausted at 
23°0' and does not exceed a width of 15’. This agreement in the 
colours gives a strong support to the diffraction-theory as above 
developed. 
A circle and a tangential curve without green are also reported 
from Boulogne sur Seine (N°. 9a). 
As regards the agreement with Hissink’s circles: the colours given 
in the table are those observed on May-19 1899 and these need 
not be identical with those of 1905. Indeed, the characteristic 
feature of diffraction-rings is that their distance is variable, depending 
as it does on the dimensions of the refracting crystal. Perbaps the 
small. remaining differences with the results of calculation may. 
herein find their explanation. Intermediate calculations gave: 
20°10' G 13,2 green-blue 
20°5’ 12,7 green-blue 
18°40! 4,2 yellow 
18°35 5,7 vellow 
The intensities of the maxima are small and the maxima are but 
little prominent. They can only become visible by the differences 
in colour and only with a very high intensity of the main circle. 
Professor VAN EVERDINGEN when asked for further information 
replied: 
“that in the observations at Zutfen, both in 1899 and 1905, the 
colours of the small circle were deseribed as very bright, as also those 
of the surrounding (circumscribed) halo or upper tangential curve”. 
It seems to me, that the above results render it extremely probable, 
that Hirssink’s circles have to be taken as diffraction-rings ; but in 
that case other similar rings must also arise by diffraction (compare 
the two cases mentioned on page 1188). 
lt is not impossible, that similar diffraction-rings may also occur 
