417 
supposing that power to cease, you will then have a batch of these undulatory 
motions travelling on and on from the source of light until they reach the 
eye, and produce upon it the sensation of light. But it is a matter of perfect 
indifference whether the cause of those undulations has in the mean time 
ceased to exist ; for, the undulations having once been excited, will travel 
through space until they reach the eye, just as the sound undulations will 
travel through the air, or the waves through water. I therefore say that 
there is no absurdity at all in the supposition that light may reach the eye 
after the star or heavenly body that emitted it has ceased to emit light— or, 
I will say, ceased to exist ; but we know nothing of its existence except by 
the light 
Dr. Irons. — Light is a vague word. 
Mr. Brooke. — The impression we derive from seeing a star at any par- 
ticular moment is just the same whether the star emits light at that moment 
or not. The star cannot affect the undulations after they are emitted 
Mr. Reddie. — May I ask this question : If you are right, how is it that 
the most distant stars dip below the horizon, just as the moon does, and do 
not continue to exhibit themselves long afterwards ? 
Mr. Brooke. — It is simply this. The stars dipped below the horizon 
long before we cease to see them. They may have dipped below the horizon 
days or weeks before 
Mr. Reddie. — Days or weeks ! If a star, say of the sixth magnitude, 
sank now, should we not cease at once to see it ? 
Mr. Brooke. — Certainly. The undulations were travelling from the star 
to us, and at length the star is in such a position that the undulations in 
that line of light no longer reach our eye, and therefore we cease to see the 
star. (Mr. Reddie : Hear, hear.) The star itself will have gone below the 
horizon long before. That light travels at a certain known rate is established 
by facts which we know astronomically, and the results which have been 
obtained with inevitable certainty appear to me to be post facto demonstra- 
tions of the truth of the theory ; because, if light had not travelled at that 
velocity many ascertained astronomical results which have followed from the 
assumption of that velocity would not have been obtained 
The Chairman. — I know that Mr. Brooke is so well acquainted with the 
subject that he can inform us whether means independently of astronomical 
observation have not been employed to prove experimentally that light does 
take a definite time to travel ? 
Mr. Brooke. —Oh, yes, there are many other means---— 
Mr. Reddie. — I have read all about the experiments you refer to ; but 
formerly they were said to “ prove ” that the velocity of light was 192,000 
miles per second, and now it is said they prove it to be 185,000 miles only 
per second. (Hear, hear.) 
Mr. Brooke. — I will now pass on to the last point to which I wish to 
allude. An observation was made with regard to the third law of motion. 
Now it is quite true that in my little work I have expressed the third law of 
motion in different terms from those used in the Princivia of Newton ; but 
2 f 2 
