98 ^ Mr. David GUI [May 23, 



clearness. Guided by the principles jnst explained, search was made 

 for comparison stars in pairs symmetrically situated with respect to 

 a Centauri, and otherwise favourably situated for measurement of 

 parallax. 



Diagram II. 



-% 



Showing comparison stars employed in determining the parallax of a Centauri. 



You will remember that from the effects of parallax all stars 

 appear to describe small ellipses about a mean position ; stars near 

 the pole of the ecliptic describing nearly circles, and those near the 

 ecliptic very elongated ellipses. Obviously, then, those pairs of stars 

 are most favourable — other conditions being equal — which lie near 

 the major axis of the parallactic ellipse. The dotted ellipse in 

 Diagram II. represents the form of the parallactic ellipse ; that is to 

 say, the form of the apparent path which a Centauri must describe if 

 it is affected by parallax. Of course the size of the ellipse is exagge- 

 rated — in fact in the diagram nearly 5000 times — therefore, remember 

 that the diagram represents only that which we can compute before 

 we have observed, viz. the shape of the ellipse, or the relations of the 

 lengths of the two axes ; the absolute size has to be determined 

 from the observations. 



The most favourable couple of comparison stars in our drawing is 

 that marked a and (3 — they are nearest to the major axis of the 

 parallactic ellipse, and they are very symmetrically situated with 

 respect to a Centauri. 



Now turn to Diagram III. Here is exhibited the results of my 



measures on a very large scale — in a manner similar to that in which 



the height of the barometer for different hours of the day, or the com- 



V. parative price of wheat at different seasons of the year or in different 



