216 THE POPULAR SCIENCE MONTHLY. 



to later ; we can leave no mark behind to denote the point in the void 

 of space the earth has quitted. Our motion round the sun is therefore 

 no help in finding its distance, and we may, in fact, for the sake of 

 simplicity in illustration, treat the earth as standing still in its orbit, 

 since the essential difficulty is thus nowise heightened. This difficulty, 

 arising from the want of a proper base-line, is similar in degree and 

 kind to that a surveyor would labor under, if he were called on to 

 measure the distance of an object of unknown size at least half a mile 

 away, without moving from his place. Success under such circum- 

 stances may well seem, not so much difficult as impossible ; yet this 

 is a fair simile of the apparent impracticability of measuring the dis- 

 tance of the sun without stepping beyond the limits of our little 

 earth, a body so small by comparison wdth the sun's remoteness that, 

 to an observer at that distance, a three-cent piece, held one hundred 

 and fifty yards from the eye, would completely cover our globe and 

 hide it from his view. 



Within such narrow bounds we must work, or not work at all, and 

 the reader, if he have not, from what he has just read, gained a defi- 

 nite conception of the principle on w T hich all such distance measure- 

 ment rests, may find aid in a very simple experiment. If any small 

 object, such as a pencil, be held in front of the eyes as near as it can 

 be conveniently seen, we may easily note the point on the opposite 

 side of the room which it appears to cover, as viewed first by the right 

 eye and then by the left. Though itself unmoved, it will appear to 

 shift its place on the wall, when the latter is distant, in a notable de- 

 gree, owing both to the difference of direction under which either eye 

 views it, and the remoteness of the background, and the amount of 

 this shifting will diminish progressively as it is carried directly away 

 from the eyes, owing to its being now seen more nearly in the same 

 direction by both, and to its approach to the wall. The change of 

 direction'^ due to the distance from the eyes only, but, this being con- 

 stant, the amount of its displacement on the wall is due only to the 

 distance of the latter, as is easily proved by walking toward it. 



The distance of the wall might. conceivably be reckoned without 

 going to it, by preparing tables which should show how this distance 

 was proportioned to the apparent motion of the pencil on it, since one 

 of these things evidently depends on the other, or which should tell 

 the distance of the pencil, by the difference of direction under which 

 we saw it. Such are the trigonometrical tables in common use, which 

 give the distance when this change of direction and place is known. 

 But this change as viewed by one eye or the other is the parallax of 

 the pencil, the known distance between the eyes being a little " base- 

 line," which plays the same part as the surveyor's longer one ; and 

 now, if we suppose ourselves in possession of tables which give the 

 distance of any object, directly its parallax is known, we may substi- 

 tute the earth for the head, two observers as far apart on it as they 



