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What follows may be taken as an attempt to get some insight 
into the quantitative relations. 
Here three different problems present themselves, which we will 
discuss successively : 
A. To what pressure of radiation is a nebula subjected from 
the stars scattered around it (system nebula-star) ? 
B. What pressure does an absorbing body (pianet) experience 
from the radiation of an extensive nebula in the neighbourhood ? 
C. Can the parts of a nebula repel each other appreciably through 
mutual radiation ? 
§ 2. The system: nebula-star. 
When we wish to make an estimation of the relation between 
the attractive forces, to which a nebula is subjected from the 
surrounding stars, and the repulsive forces caused by the radiation 
of these same stars, we may begin by remarking that it is independent 
of the scattering in space of the stars considered. For the two forces 
are in the ratio r-? hence their ratio is not influenced by the 
distance. As not all the stars of the same absolute magnitude have the 
same mass, it would practically be necessary for the determination 
of the resultant of the active forces to know the nature of each of 
the stars concerned accurately. This is, of course, impossible. In 
our investigation we shall assume that on an average all the stars 
have an equally large mass, and radiate equally strongly as our sun. 
On this simplified supposition the ratio of the attraction of the whole 
stellar system to the repulsion caused by the radiation of the same 
system, is equal to that of the same forces exerted by one star at 
any distance. 
With a view to the hypothesis of KaAPTEYN and CAMPBELL men- 
tioned before ') we will examine the following case more closely. 
A star with a mass equal to that of the sun may be at1 parsec. 
distance from a spherical®) nebulous mass of a radius of 15000 
astronomical units. Seen from the star, the nebula occupies the 
0.0014» part of the sky. 
Let us assume in order to find an upper limit of the pressure of 
radiation to which the nebula is subjected, that all the radiation 
received from the star, is absorbed. (We know that in reality the 
absorbed fraction is exceedingly small). The star emits as much 
1) Compare also the view of H. Suaptey, Astrophys. J. 50, (373), 1919. 
2) I choose the spherical form to simplify the. calculations; one should not think 
here of a planetary nebula, which is known to show on the other hand very 
quick proper motion. 
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Proceedings Royal Acad. Amsterdam. Vol XXIV. 
