CHAPTERS ON THE STARS. 235 



If the practice of separating the visible stars into only six orders 

 of magnitude were continued without change, we should still have the 

 anomaly of including in one class stars of markedly different degrees of 

 brightness. Some more than twice as bright as others would be desig- 

 nated of the same magnitude. Hence, to give quantitative exactness 

 to the results, a magnitude is regarded as a quantity which may have 

 any value whatever, and may be expressed by decimals — tenths or even 

 hundredths. Thus, we may have stars of magnitude 5.0, 5.1, 5.2, etc., 

 or we may even subdivide yet farther and speak of stars having 

 magnitudes 5.11, 5.12, etc. Unfortunately, however, there is as yet 

 no way known of determining the amount of light received from a 

 star except by an estimate of its effect upon the eye. Two stars are 

 regarded as equal when they appear to the eye of equal brilliancy. 

 In such a case the judgment is very uncertain. Hence, observers have 

 endeavored to give greater precision to it by the use of photometers, — 

 instruments for measuring quantities of light. But even with this 

 instrument the observer must depend upon an estimated equality of 

 light as judged by the eye. The light from one star is increased or 

 diminished in a known proportion until it appears equal to that of 

 another star, which may be an artificial one produced by the flame of a 

 candle. The proportion of increase or diminution shows the difference 

 of magnitude between the two stars. 



As we proceed to place the subject of photometric measures of star 

 light on this precise basis we find the problem to be a complex one. 

 In the first place not all the rays which come from a star are visible 

 to our eyes as light. But all the radiance, visible or invisible, may be 

 absorbed by a dark surface, and will then show its effect by heating that 

 surface. The most perfect measure of the radiance of a star would 

 therefore be the amount of heat which it conveys, because this expresses 

 what is going on in the body better than the amount of visible light 

 can do. But unfortunately the heating effect of the rays from a star 

 is far below what can be measured or even indicated by any known 

 instrument. We are therefore obliged to abandon any thought of 

 determining the total amount of radiation and confine ourselves to that 

 portion which we call light. 



Here, when we aim at precision, we find that light, as we under- 

 stand it, is properly measured only by its effect on the optic nerve, and 

 there is no way of measuring this effect except by estimation. Thus, all 

 the photometer can do is to give us the means of increasing or dimin- 

 ishing the light from one star, so that we can make it equal by estima- 

 tion to that from some other star or source of light. 



The difficulty of reaching strict results in this way is increased by 

 the fact that stars are different in color. Two lights can be estimated 

 as equal with greater precision when they are of the same color than 



