TRANSACTIONS 
OF THE 
ROYAL SOCIETY OF SOUTH AFRICA. 
VOL. II. 
A PEBLIMINAEY DETEEMINATION OF THE ABSOEPTION 
OF LIGHT BY THE EAETH'S ATMOSPHEEE. 
By a. W. Eobeets, D.Sc, F.E.A.S., F.E.S.E., F.E.S.S.Af. 
(Eead October 20, 1909.) 
In almost all stellar photometric work, a knowledge of the amount of 
light absorbed by the Earth's atmosphere is necessary. In certain very 
refined variable star work, as accurate a determination as possible of this 
absorbing quality of the atmosphere is desirable. 
It is known to all that if there were no atmosphere surrounding the 
Earth the stars would appear brighter. The air acts as an obscuring veil 
dimming their lustre. 
The thicker the quantity of air through which the observer views the 
stms the greater will be their diminution in brightness. 
She minimum dimimution takes place when the star being observed is 
in the zenith ; the maximum when it is on the horizon. 
The amount of light lost by a star when in the zenith is called the 
coefficient of atmospheric absorption. It is the difference in brightness 
between what the star would be if there were no air, and what it is when 
viewed through about two hundred miles of air. 
Once this coefficient is known, it is possible to determine the loss of 
light at any altitude, always assuming that the atmosphere is homo- 
geneous in structure. 
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