14 SIR ROBERT 8. BALL, LL.D., F.R.S., ON 
several brilliant lmes crossmg the spectrum. Here we get 
nature to speak for herself—we are looking at the actual 
photographs. In these days there is no excuse for giving 
illustrations which are not absolute photographs.* 
The spectroscope shows in Nova Persei the presence of a 
great mass of blazing incandescent hydrogen, and when you 
accompany this with ‘the fact that the ‘star suddenly broke out 
and declined again, it becomes of great interest indeed. [ 
pointed out this statement how in the spectrum of this star 
the dark lines and bright lines of hydrogen are close side by 
side. It would take too long to go into more detail; but 
there js no doubt, from the evidence that these photographs 
contain, that there were two bodies concerned. Those two 
bodies were moving with different velocities. The dark line 
belongs to one and the bright line (speaking generally) may 
be said to belong to the other. Taking that imto account, 
and taking into account the suddenness of its outbreak, and 
the indications of blazing hydrogen, it is quite easy for us to 
form a supposition, which is not an unnatural one, as to what 
was the cause of that remarkable star. I mentioned that, of 
the stars above us, the dark stars are in all probability 
incomparably more numerous than the brilliant ones. We 
do not see those dark stars under ordinary circumstances ; 
but, in their myriads, it does sometimes happen that one of 
those dark stars, hurrying along, comes into ccllision with 
another. I have no doubt that the occurrence of such a 
collision is excessively rare. We must remember that these 
bodies are moving at enormously high velocities in vast 
numbers, and there is just a possibility of two of them 
striking. It is not unlike supposmg that two rifle shots fired 
at random, in the air, should strike in the course of their 
fight. I need not tell you that if one man were firing a 
rifle in one place and another man a mile away were firing 
in an opposite direction, that it is most unlikely the two 
bullets should strike each other. If myriads of rifle 
bullets were being fired in every sort of direction then it 
would be conceivable that some pair of those bullets would 
strike each other. It is not so very unlikely—in fact there 
is the very best reason for believing that it may sometimes 
happen. I believe it is recorded that on a field of battle one 
rifle bullet has been known to pierce another. I think I have 
* Hor spectra of Nova Persei, 1901, see “Observations on the star of 
Nova Persei,” by Sir Norman Locky er, Proc. Royal Society for Feb. 28, 
March 7, and June 20, 1901, 
