ON ASTRONOMY. 



133 



metric measurements, concluded thatSirius must be 140,000 times more 

 distant than the sun. But even this is much within the true distance. 



But let us glance at the actual measurements which have enabled 

 the astronomer, in some few cases at least, to sound the amazing depths 

 of the heavens. 



The star j Draconis, of Flamsteed's catalogue, has a very large 

 proper motion, amounting to nearly 5" annually. Under the suppo- 

 sition that those stars which have the greatest proper motion are 

 nearest to us, and therefore most favorable to researches on the par- 

 allax, this star was selected by Dr. Bradley for special observations, 

 with a view to detect this element. It had also the great advantage 

 of passing near the zenith of the Wansted observatory, by which the 

 troublesome irregularities of refraction would be avoided. He failed 

 to detect any parallax, but his labor was rewarded by the beautiful 

 discovery of the aberration of light. Bradley depended upon finding 

 a variation in the elements of right ascension and declination. But 

 this is by no means the most delicate test which can be applied for the 

 determination of a minute annual change in the position of the star. 



Galileo was the first to suggest that where a large and a small star 

 are very close together the smaller star might appear faint from being 

 placed at a vastly greater distance than the larger. If this were not 

 always the case, of which there is no probability, it would be likely to 

 occur in some instances. Wherever this holds good the stars will 

 undergo an annual change in their relative positions, either approach- 

 ing and receding from each other^ or the one revolving about the other. 

 Thus, in figure 19, if we suppose s and s' to be two stars situated in a 

 right line from the sun and in the direction of the pole of the ecliptic, 

 as the earth goes round in her orbit A E B the star s may appear to 

 revolve round s', and it must do so if the radius of the earth's orbit 

 holds any assignable ratio to the distance of the nearer star. Thus, 

 when the earth is at A, s will appear at a in the distant heavens; 

 when the earth is at E the star will appear at e; the earth at B will 

 carry the star to h ; and hence while the earth goes round in the orbit 

 A E B the star S will appear to go round 6' in the curve a eh. No 

 instance has been discovered where this idea has been fully realized. 

 Nor indeed could any available results be obtained from it until a 

 very late period, for the want of instruments fitted to measure so small 

 an angle as the largest annual parallax. The micrometer in its dif- 

 ferent ibrms supplies this want. The manner in which very minute 

 angles are measured by this appendage to the telescope will be under- 

 stood by a reference to figure 22, 

 which is a rectangular frame 

 with a fine screw at each end 

 passing tlirough the frame and 

 attached to sliding pieces within 

 it. These sliding pieces carry 

 the spider lines a and 6, which 

 by turning the screws are made 



to approach or separate from each other. Graduated circles are 

 fixed to the heads of the screws. Suppose one complete revo- 

 lution of the screw changes the position of the spider lines by 2' or 



Fic^ 22 



