36 AN ESSAY ON 



the heads of the two pins or needles will con~ 

 stantly appear as one in the common axis. 

 When the heads, however, are near to the eyes, 

 this experiment is not so satisfactory as the 

 former, since, in these positions, they seem as 

 broad transparent shadows, for reasons known 

 to every person a little conversant in optics; 

 whereas the holes appear well defined, though 

 almost touching us. Again ; if we hold two 

 thin rulers in such a manner, that their sharp 

 edges shall be in the optic axes, one in each, or 

 rather a little below them, the two edges will 

 be seen united in the common axis, and this 

 apparent edge will seem of the same length with 

 that of either of the real edges, when seen alone 

 by the eye in the axis of which it is placed. If 

 instead of two rulers we employ two strings of 

 different colours, as red and green, the like unity 

 of appearance will be observed. But in this 

 experiment it frequently happens, that, contrary 

 to what we might naturally expect, only one of 

 the strings is seen at a time. When, however, 

 only one is seen, its apparent situation is exactly 

 the same as that of the string, compounded, if 

 I may so express myself, of the two when seen 

 together; and hence we have a convincing 

 proof, if any were wanted, that the single ap- 

 pearances of objects must depend upon some 



