( 1268 ) 
A point M somewhere in the | 
atmosphere of the sun will be seen ie 
on the centre of the disk by an 
observer stationed on the line MA; . 4 
= 5 ha ad 
but an observer on MB will see it SoM 
not far from the limb. To the second y 
observer the region round M appears 
much less bright than it does to A 
the first one. This proves, that M Mi Been he 
receives less light (perhaps only half 
as much) along the direction 5M Fig. 2. 
than along af. How, in a point J/, the intensity of the irradiation 
by a definite kind of light depends on the direction of incidence, 
can easily be found, provided that we know the average distribution 
of the brightness on the solar disk, for the light in question. 
In fig. 3 PQ shows the gradual 
UE decrease of the brightness from 
mA | | the centre C toward the limb R 
iy . : ; 
AE | of the solar disk for waves be- 
Vi | al tween 405 and 412 uu, accord- 
0, | ing to speetro-photometric mea- 
R n Y C ; 1 V7 as 
| tA, surements by H. C. Vocert’). Let 
| A 4 
ig RNC" represent a section of the 
| | | photospherie surface, and suppose 
\ | | (OA an observer to be placed at a 
AS | | 
| 
€ 
| 
ME aa UL great distance in the direction 
ZV ‚P wt: 
era SI fl 
CC’; then it is clear that e.g. 
from .V — that is, in a direction 
making an angle ANB (= NCC’ 
= 4) with the normal to the 
photosphere, — an average quantity 
of light proportional to the ordi- 
nate mn of the P’ Q’-curve, emer= 
ges from the solar atmosphere. 
Fig. 3. We now define a point m’ on 
the radius vector CN by making Cm'= mn, and do the same on 
the other radii of the section RNC’. Thus acurve PQ’ is obtained, 
representing the transmissive power of the solar atmosphere for the 
selected kind of light, as a function of the angle of emergence g. 
With the aid of this figure we may now proceed to the construction 
1) H. CG. Voegen, Ber. der Berl. Akad., 1877, S. 104, 
