306 ANNUAL EEPOBT SMITHSONIAN INSTITUTION, 1908. 



still hardly sufficient for the purpose, that the whole of the solar sys- 

 tem is moving through space in the direction toward the constellation 

 of Hercules. Later observations and computations have confirmed 

 Herschel's conclusions, and we have even been able of late to fix with 

 some precision the velocity of this motion, which amounts to 20 

 kilometers per second. This velocity is a fifteen-thousandth part of 

 the velocity of light. In the one hundred and fifty years elapsed 

 since Bradley determined for the first time the position of numerous 

 stars with modern precision, the solar system must thus have covered 

 a distance of exactly a hundredth part of a light year — i. e., we are thus 

 enabled to make pictures of the sky as seen from points of view at a 

 mutual distance of a hundredth of a light year. Our eye distance of 

 sixteen light minutes is thus increased more than three hundred fold. 

 True, this distance falls still considerably short of that adopted by 

 Heath, but it appears that, for a considerable part of the stars, it is, 

 though not nearly so great as might be desired, still in a certain way 

 sufficient. 



IMPOSSIBILITY OF DETERMINING DISTANCE OF INDIVIDUAL. STARS. 



There is, however, a difficulty in the way, which prevents our pic- 

 tures from giving a stereoscopic view of the stars at all, and thus 

 prevents the determination of the distance of any star in this manner. 

 The difficulty is that the changed directions in which, after the lapse 

 of one hundred and fifty years, we see the stars, is not exclusively the 

 consequence of the sun's motion through space, but is due also to a 

 real motion of the stars themselves. The two causes of displacement 

 which, in the case that we take the diameter of the earth's orbit as 

 eye-distance, are separable by means of a simple device, become 

 inseparable in the present case. 



In order to see whether this difficulty be or be not absolutely in- 

 superable, I will take a parallel case on the earth. 



DISTANCE OF INSECT CLOUD. 



At a certain distance we observe a cloud of insects hovering over a 

 small pond. In order to evaluate the distance separating the insects 

 from our eye, suppose that we make a photograph ; then, after a few 

 seconds, a second one from a slightly different standpoint. It must be 

 evident that even if we have used an instrument which clearly shows 

 the individual insects, the two pictures put in the stereoscope will not 

 furnish a stereoscopic view of them individually; on the contrary, 

 the picture as seen in the stereoscope will be perfectly chaotic. The 

 reason, of course, is that in the interval between the taking of the two 

 photogTaphs the insects have moved. Does it follow that an evalu- 

 ation of the distance can be obtained? 



