STKUCTUEE OF THE UNIVERSE — KAPTEYN, 305 



the Royai Observatory at Greenwich. Placing the two photographs 

 side by side in the stereoscope, we shall clearly see the moon " hanging 

 in space," and may evaluate its distance. 



But already for the sun and the nearest planets, our next neighbors 

 in the universe after the moon, the difficulty recommences. 



The reason is that any available distance on the earth, taken as 

 eye distance, is rather small for the purpose. However, owing to 

 incredible perseverance and skill of several observers and by substi- 

 tuting the most refined measurement for stereoscopic examination, 

 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 the 

 planetary distances. 



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

 of times farther 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 a 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 sixteen 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 photographs we may 

 perhaps get somewhat beyond what we can attain by simple stereo- 

 scopic inspection, but, as we said a moment ago, astronomers have not 

 succeeded in this way in determining 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, INIr. 

 Heath, constructed, making use of the data obtained in the way pres- 

 ently 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. 



MOTION OF SOLAR SYSTEM THROUGH SPACE. 



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

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

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



