lO-i SYDNEY CHAPMAN, B.A., D.SC, F.R.A.S.^ ON 



8ome fainter than the average brightness of the magnitude. 

 JSTevertheless the classification is quite a good working one, and 

 scientific in its principle. 



A star is visible to us by the light which we receive from it, 

 and this is focussed by the lens of our eye upon the retina. 

 The impression which the light makes on the retina depends 

 upon the brig'ntness of the beam and its diameter as it enters 

 the eye. Now the pupil of the eye is not always of the same 

 diameter ; when we go into a dark place the pupil enlarges, so 

 that more light can enter the eye ; thus the amount of light of 

 any given briglitness that enters the eye may vary a little, and 

 the impression which the light makes upon us depends upon 

 the two conditions, of the quantity of light entering the eye 

 and its brightness. Since, however, the pupil of the eye does 

 not vary very much in diameter, we must resort to other means 

 if we wish to perceive the light of stars that are intrinsically 

 too faint to produce a sensible impression upon us. If we 

 could enlarge the pupil of the eye indefinitely, stars fainter 

 than the 6th magnitude w^ould no doubt become visible. 

 This is, of course, impossible in itself, but by means of the 

 telescope, we can greatly increase the beam of light from the 

 star which can be collected in the eye and focussed on the 

 retina, so that the telescope enables much fainter stars to be 

 seen than could possibly be viewed by the eye without sucli 

 assistance. 



Every increase in the size of telescopes hitherto has led to 

 an increase in the number of stars rendered visible to us, but 

 in spite of the great advances that have already been made in 

 telescopic power, not even our largest telescope has sufficient 

 liglit-gathering power to reveal to us half the number of the 

 stars which undoubtedly exist. 



Before dealing with the question of the relation between the 

 size of the telescope and the faintness of the stars which we 

 desire that it should reveal to us, it is necessary to say a few 

 words as to the detinition of stellar magnitudes, that classifica- 

 tion of stars according; to their brio-litness which has been already 

 mentioned. 



The lioht of two candles taken tof^ether is of course twice the 

 amount given by one, and the excess of tlie light of the two 

 over that of one corresponds to a definite difference in the 

 brightness of the two. If now a third candle be added to the 

 two, the increase in the <ivantity of light equals the increase in 

 the light when a second is added to a single candle, but the 

 increase in hrujlitness is less in the former case than in the case 



