PARALLAX. 483 



Parallax is simply the difference between the directions of an object 

 when seen from two different positions. Now we can illustrate this by a 

 very simple method, which we have often tried as a " trick," but which has 

 been very happily used by Professor Airy to illustrate the doctrine of 

 parallax. We give the extract in his own words : 



" If you place your head in a corner of a room, or on a high-backed 

 chair, and if you close one eye and allow another person to put a lighted 

 candle upon a table, and if you then try to snuff the candle with one eye 



Fig. 522. Works of a clock. 



shut, you will find you cannot do it. ... You will hold the snuffers too 

 near or too distant you cannot form any idea of the distance. But if you 

 open the other eye, or if you move your head sensibly you are enabled to 

 judge of the distance." The difference of direction between the eyes, which 

 is so well known to all, is reaily a parallax. It can also be illustrated by 

 the diagram herewith 



If two persons, A and C (fig. 523), from different stations, observe the 

 same point, M, the visual lines naturally meet in 

 the point, M, and form an angle, which is called 



c ' js the angle of parallax. If the eye were at M, 



this angle would be the angle of vision, or the 

 angle under which the base line, A C, of the two 



observers appears to the eye. The angle at M also expresses the apparent 

 magnitude of A C when viewed from M, and this apparent magnitude is 

 called the parallax of M. 



Let M represent the moon, c the centre of the earth represented by the 

 circle, then A c is the parallax of the moon ; that is, the apparent magnitude 

 the semi-diameter of the earth would have if seen from the moon. If the 

 moon be observed at the same time from A, being then on the horizon, and 

 from the point, B, being then in the zenith, and the visual line of which when 

 extended passes through the centre of the earth, we obtain, by uniting the 

 points, A C M, by lines, the triangle, A C M. 



