304 Professor Dr. J. C. Kapteijn [May 22, 



astronomers have succeeded in overcoming the difficulty for the sun. 

 I think we may say that at present we know its distance to within a 

 thousandth part of its amount. Knowing the sun's distance we get 

 that of all the planets by a well-known relation existing between tlie 

 planetary distances. 



But now for the fixed stars, which must be hundreds of thousands 

 of times further removed than the sun. There evidently can be no 

 question of any sufficient eye-distance on our earth. Meanwhile our 

 success with the sun has provided us with i^ new eye-distance, 24,000 

 times greater than any possible eye-distance on the earth. For now 

 that we know the distance at which the earth travels in its orbit 

 round the sun, we can take the diameter of its orbit as our eye- 

 distance. Photographs taken at epochs six months apart will repre- 

 sent the stellar world as seen from points the distance between which 

 is already best expressed in the time it would take light to traverse it. 

 The time would be about 1(5 minutes. 



However, even this distance, immense as it is, is on the whole 

 inadequate for obtaining a stereoscopic view of the stars. It is only 

 in quite exceptional cases that photographs on a large scale — that 

 is, obtained by the aid of big telescopes — show any stereoscopic 

 effect for fixed stars. By accurate measurement of the photos we 

 may perhaps get somewhat beyond what we can attain by simple 

 stereoscopic inspection, but, as we said a moment ago, astronomers 

 have not succeeded in this way in detei'mining the distance of 

 more than a hundred stars in all. 



How far we are still from getting good stereoscopic views appears 

 clearly from the stereoscopic maps which your countryman, Mr. 

 Heath, constructed, making use of the data ol^tained in the way 

 presently to be considered. In order to get really good pictures he 

 found it necessary to increase the eye-distance furnished by the 

 earth's orbit 19,000 times. 



Are there, then, no means of still increasing this eye-distance ? 



Motion of Solar System through Space. 



There is one way, but it is a rather imperfect one. Sir WiUiam 

 Herschel has been the first to show, though certainly his data were 

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

 system is moving through space in the direction towards the con- 

 stellation 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 kilometres per second. This velocity is a 15,000th part of the 

 velocity of light. In the 150 years elapsed since Bradley determined 

 for the first time the position of numerous stars with modern pre- 

 cision, the solar system must thus have covered a distance of exactly 

 ^ hundredth part of a light-year, i.e. we are thus enabled to make 



