114 NEW MEASUEEMENTS OF DISTANCE OF SUN. 



required that there shall be no sudden jumps in the errors exceeding- 

 one or two hundredths of a second of arc. 



Now, when one comes to face a problem like this, one must inquire 

 very carefully what is the real accuracy of the photograph. There is 

 no doubt that the ordinary photographic telescope properly worked 

 will repeat itself yery well; it Avill take two plates o.f the same region 

 which agree wdth one another excellently. But the question is, How 

 would two plates of the same region taken with diiferent telesco})es 

 agree? We know that indiyidual observers have peculiarities of 

 their own which they can rej^eat almost ad infinitum. Does a photo- 

 graphic telescope do the same, or has it at last conquered that bad 

 habit of idiosyncrasy which has made so much trouble in all refined 

 astronomical work of the older kinds? "When we started on the 

 Eros campaign there was practically no information to be obtained 

 upon this point. Almost all the photographic telescopes at work had 

 been engaged upon their own zone of the chart, and almost nothing 

 was known of how the results from different instruments would com- 

 bine. But in our parallax problem this question is fundamental, and 

 must be answered as soon as possible. I therefore ventured to 

 propose to myself to undertake the reduction of a small section of 

 the photographic results, for a period of nine clays in November, 

 1900, having before me two objects: Firstly, to discover how far it is 

 possible to combine photographs taken at different observatories, how 

 far, in fact, photographic telescopes are giving really accurate results 

 or merely reproducing their own errors; secondly, to obtain a pro- 

 visional value of the solar parallax, with a probable error if possible as 

 small as that found by Sir David Gill with the heliometer, and to find 

 out provisionally whether Eros was going to confirm that result or to 

 join in the secession from the adopted value. 



Perhaps I may venture to think that the results of this enterprise 

 have lieen in some measure successful. I found that as a general 

 rule the results from different telescopes do not combine directly as 

 well as could be hoped and that there are many precautions which 

 must be taken in using them if we are to avoid serious systematic 

 error and a ruination of the parallax determination; but I believe 

 that it is possible to avoid these difficulties and that the photograj^hs 

 properly treated will give a determination of the parallax of far 

 greater accuracy than has hitherto been obtained. I found also that 

 the 300 exposures in that period of nine days, contributed altogether 

 l)y nine different observatories, gave a value of the solar parallax, 

 8 "797 seconds ± -001:7 second, which is in such nice agreement with 

 Gill's 8 -802 seconds ± -005 second that one may feel in one's heart 

 (though of course must never express the feeling so prematurely as 

 this) some hope that, in adopting 8 "80 seconds as the official value 

 of the solar parallax, the conference of 189G was not so wrong as 

 some people have been prepared to believe. 



