( 2 3 / f 7 ) 
made with the R-light and the V-light of a suitable line 1 ), should 
show the above difference very distinctly. 
The spectroscope will reveal the following particulars. 
Suppose the slit to be so placed in the sun’s image, that it bisects 
the spot and coincides with a radius of the disk. In fig. 6 PC be 
a Fraunhofer line; P points to the periphery, C to the centre. The 
horizontal bands indicate the spectrum of the umbra and the pen¬ 
umbra as shown with light not undergoing anomalous refraction. 
With the R-light the umbra is greater and extends chiefly towards 
P (cf. fig. 4) ; so in the upper half of the spectrum the Fraunhofer 
line appears to be shifted to the red, and more markedly still, be¬ 
cause at the same place in the spot the Y-light attains its greatest 
brightness (cf. the point p in fig. 5). In the lower half of the spec¬ 
trum R-light and Y-light have changed parts *). 
The widening of the line extends beyond the average penumbra 
because with R- and Y-light the penumbra too is greater than with 
waves not subject to anomalous dispersion. 
We conclude that in the spectrum of an excentrical spot — the 
slit bisecting that spot in the direction of the centre of the disk — 
the Fraunhofer lines must be more or less curved in the shape of 
the letter S, to a degree, depending on the intensity of the spot’s 
distance from the centre of the disk. Whenever that distance is so 
*) Perhaps some iron line will serve the purpose better than one of the winged 
calcium or hydrogen lines, whose refraction effects might be confusingly great. 
2 ) The distribution of the light in fig. 6 has been deduced from that in figures 
* and 5, in which process the shape of the dispersion curve, as given in fig. 8, 
ls paid due attention to. 
Indeed, one must imagine the whole spot of fig. 4 to appear smaller and smaller 
as the waves we are considering recede from the absorption line, because in fig. 8 
the ordinates (R m a,„)r gradually decrease towards the red, until they reach the 
constant value R 0 A 0 (cf. p. 281). The distributions of the light along a cut PC 
through fig. 4, such as they would successively appear with increasing wave-lengths, 
were plotted side by side in fig. 6, going from the absorption line to the red. 
On the violet side of the line one has to proceed in the same way. There the 
negative ordinates of fig. 8 decrease as we recede from the line, and the distribution 
°f the light prevailing along a cut PC through fig. 5, must be transferred to fig. 6 
on a smaller and smaller scale. But in the point c' of fig. 8 we have (R m )v = 0; 
bght of that wave-length suffers no refraction at all in the solar atmosphere, so that 
l he spot-spectrum is here interrupted. Next, (Rm )v attains positive values, in¬ 
creasing up to R 0 a 0 ; the distribution of the light has again to be taken from fig. 4. 
So the spot-lines are not only inclined, but also asymmetric, a feature which 
especially the weaker lines will show, provided that the dispersion of the spectral 
apparatus be sufficiently high. 
19 
Proceedings Royal Acad. Amsterdam. Vol. XII. 
