100 Mr. David Gill [May 23, 



years, is now exhibited in the daily papers. Imagine the star a about 

 a mile immediately below any point of that curve, and the star /3 

 rather over three-quarters of a mile immediately above the same point, 

 and you would then have a diagram to scale.* The middle horizontal 

 line represents the mean difference of these two distances, and each 

 dot or mark on Fig. 1 of the diagram represents the variation of that 

 distance according to each successive observation. The different 

 kinds of dot represent measures made at different hour angles, or 

 when the relation of the direction of measurement to the line joining 

 the observer's eye is different. These different kinds of personal 

 errors were separately investigated, and they were then allo^ved for 

 and the observations were corrected accordingly. 



The observations so corrected are represented in Fig. 2, where 

 each black dot expresses the result of the observations of a single 

 night, and the curve is the commuted curve resulting from the 

 mathematical discussion of the observations. 



You must be careful to understand that this is not simply the kind 

 of curve which best represents the observations. The curve is limited 

 by purely geometrical conditions to have its maximum on March 7, and 

 its minimum on September 10, and to follow a precise form of curve 

 according to a simple law. The observations only determine the 

 range from maximum to minimum, and yet you see how perfectly the 

 maximum of the observations agrees with the maximum of the curve, 

 and the minimum of the observations with the minimum of the curve, 

 and how closely the law is followed throughout. 



The result was that from these observations the parallax of a 

 Centauri was 0" • 747, or practically three-quarters of a second of arc. 



But I was not content with this result alone. I wished further 

 confirmation, and selected another pair of stars, a and /3', shown in 

 Diagram II. 



From similar observations with these comparison stars I obtained 

 for the parallax of a Centauri 0" • 760, a result which is identical 

 with the last within the limits of the probable error of either. 



My friend Dr. Elkin selected the stars a h and a b' as his com- 

 parison stars, and in a precisely similar way he obtained as the mean 

 of his results a parallax of 0"*762, a result identical with my own, so 

 that we may conclude as one of the most certainly established facts 

 of astronomy that the parallax of a Centauri relative to an average 

 star of the seventh or eighth magnitude is three-quarters of a second 

 of arc. 



It is therefore beyond all doubt that Henderson's discovery was a 

 real one. Herschel's verdict must therefore bo confirmed, and the 

 palm for first breaking down the barriers tliat separated us from any 

 knowledge of the distances of the fixed stars be accorded to the 

 memory of the Cape Astronomer, Henderson. 



In the wall diagram one second of arc wns represented by about fifteen inches. 



