THE NUMBEJ5 OF THE STARS. 



107 



their aid. The eye registers a momentary im]3ression ; the 

 photographic plate registers a cumulative effect. The difference 

 loetween the two can be illustrated by imagining that leaden 

 bullets are being dropped upon a spring balance ; if the scale 

 pan is flat, each ballet, as it falls, just depresses the spring, and 

 then rolls off' immediately, and the scale pan rises again. If 

 the bullets are falling in a continuous stream, the balance will 

 show a constant small depression of the spring. This last 

 condition is analogous to the impression of Hght on the eye, 

 but if the scale pan is in the form of a cup, so that it collects 

 and holds the bullets, instead of allowing them to roll off, then, 

 as more and more bullets fall into it, the scale pan sinks lower 

 and lower., and this offers us an analogy to the cumulative 

 action of light on a photographic plate. If particles of dust 

 were slowly falling on the scale pan of a spring balance, the 

 balance would show no appreciable depression due to any one 

 particle, but after a long time enough dust would collect to 

 produce an evident and measurable depression. This example 

 may illustrate how it is possible to photograph very faint stars 

 with the telescope, because the photographic plate accumulates 

 the effect of a constant stream of faint light, concentrated upon 

 it by the lens, and after a long time euables the impression 

 to be recorded of stars far too faint ever to be seen directly 

 by eye with the same telescope ; the telescope registers 

 the effect of the light accumulated over a long period of time, 

 while the eye can only register the impression of the moment. 

 Thus the photograpliic plate can supply us with the images of 

 stars one hundred times as faint as the faintest wduch can be 

 seen directly by the eye with the same telescope ; that is to 

 say, photography adds five magnitudes to our power of detecting 

 faint stars. 



The above table gives the number of stars of each mao-nitude 

 from the very brightest down to the 20t]i magnitude, those 

 visible to the naked eye beiug grouped under magnitudes 1 to 6. 

 There is souie difference between the scale of magnitude for 

 stars as registered on photograpJis froui the scale adopted for 

 observations made directly by the eye, in other words the 

 visual and the photographic magnitude scales are not identical. 

 The difference between the two scales does not, however, affect 

 the general principle, but whereas there are .'),150 stars down 

 to the Gth magnitude on tlie photographic magnitude scale, 

 there are in actuality some 6,000 stars visible to the naked eye. 

 This means that the limit for naked eye observations stands at 

 about the magnitude 6 J. "Of these 6,000 stars, of course only 



