149 



contains y. Of this coefficient we can say nothing withont making 

 special hypotheses. 



§ 12. Finally vve shall shortly discuss the question whether the 

 width of Fraunhofrk's lines in the speclrum of the snn can teach 

 us something about the quantity of the absorbing vapour which 

 produces them. Let tis consider an arbitrarily chosen rather fine line, 

 the calcium line A 5868. Its width is certainly smaller than 0,1 A.U., 

 by whicli I mean that, 0,05 A.U. from the middle, the intensity of 



the light amounts to more than the part - of that which is seen at 



e 



a small distance from the line and which would exist in the place 

 of the line itself if no calcium vapour were present. 



If (f is the thickness of the traversed layer of calcium vapour we 

 may write, giving to i' the value that corresponds to the above 

 mentioned distance of 0,05 A.U. 



2M < 1 , 

 so that 



'±\^ 

 >>,^< ke'"' . 



We can calculate the right hand side of this inequality if we 

 make an assumption concerning the temperature J" of the ab.sorbing 

 layer. For 7'=(ï000° we find in this way ho<f<i7,0 and for 

 T=:3000° /t„d<98. 



Now, if it were allowed to use the formula (26), this upper limit 

 for /i/t would lead to a similar one for pd. We should have for 

 r=6000°, pd< 0,0015 and for 7' = 3000°, ^^d < 0,0074. As /> 

 represents the pressure expressed in mm. of mercury, whereas ö is 

 expressed in cm., we might infer from these numbers that the quantity 

 of calcium vapour which produces the line in question is very small. 

 Some reserve however must be made here. It may very well be 

 that a small part only of the calcium atoms take part in the absorp- 

 tion. Then the above inequalities will still hold, provided we 

 understand by /> the pressure of the "active" vapour. If we mean 

 by p the total pressure of the calcium vapour present we should 

 have to multiply the given numbers by 10', if one ten millionth 

 part of the atoms were active (coinp. ^11). For the first temperature 

 this Avould give />fi< 15000 and for the second p(f< 74000. The 

 last of these numbers corresponds e. g. to a thickness of 0.75 km. 

 if the pressure is 1 mm. of mercury. 



If we wish to abstain from all siqipositions on the nature of the 



11 



Proceedings Royal Acad. Ainslerdam. Vol. XVIII. 



