72 PLEASANT WA YS IN SCIENCE. 



distance seems very great by comparison with Mars's occa- 

 sional opposition distance of 35 million miles, yet there are 

 two points in which asteroids have the advantage over 

 Mars. First, they are many, and several among them can 

 be observed under favourable circumstances ; and in the 

 multitude of observations there is safety. In the second 

 place, which is the great and characteristic good quality of 

 this method of determining the sun's distance, they do not 

 present a disc, like the planet Mars, but a small star-like 

 point. When we consider the qualities of the heliorr.etric 

 method of measuring the apparent distance between 

 celestial objects, the advantage of points of light over discs 

 will be obvious. If we are measuring the apparent distance 

 between Mars and a star, we must, by shifting the movable 

 object-glass, bring the star's image into apparent contact 

 with the disc-image of Mars, first on one side and then on 

 the other, taking the mean for the distance between the 

 centres. Whereas, when we determine the distance between 

 a star and an asteroid, we have to bring two star-like points 

 (one a star, the other the asteroid) into apparent coincidence. 

 We can do this in two ways, making the result so much the 

 more accurate. For consider what we have in the field of 

 view when the two halves of the object-glass coincide. 

 There is the asteroid and close by there is the star whose 

 distance we seek to determine in order to ascertain the 

 position of the asteroid on the celestial sphere. When the 

 movable half is shifted, the two images of star and asteroid 

 separate; and by an adjustment they can be made to 

 separate along the line connecting them. Suppose, then, 

 we first make the movable image of the asteroid travel away 

 from the fixed image (meaning by movable and fixed images, 

 respectively, those given by the movable and fixed halves of 

 the object-glass), towards the fixed image of the star the 

 two points, like images, being brought into coincidence, we 

 have the measure of the distance between star and asteroid. 

 Now reverse the movement, carrying back the movable images 

 of the asteroid and star till they coincide again with theii 



