40 AN ESSAY ON 



axis of the eye, by which it is not seen. As I 

 have supposed the distance between the outer 

 holes to be adapted to the interval of the eyes 

 when they are directed to a very remote object, 

 the optic axes may, in this case, be regarded as 

 parallel to each other. The object, therefore, 

 will still be seen through those holes, though 

 the distance of the card from the eyes be con- 

 siderably varied ; and at all the different di- 

 stances, the same appearances will be observed, 

 as those which have been mentioned. 



Again ; take three strings of different colours, 

 as red, yellow, and'green, and fasten, by means 

 of a pin, one end of each to the same point of a 

 table. Place now their loose ends in such a 

 manner, that when you look at the pin with 

 both eyes, the visual base being parallel to the 

 edge of the table, the red string may lie in the 

 axis of the right eye, the green in that of the 

 left, and the yellow in the common axis. When 

 things are thus disposed, and both eyes are di- 

 rected to the pin, the red and green strings, in- 

 stead of appearing separate, each in one of the 

 optic axes, and inclined to the visual base or 

 edge of the table, will now be seen occupying 

 but one place, either together or successively, 

 as was formerly mentioned, and at right angles 

 to the visual base, or edge of the table ; in short, 

 exactly in the situation, which the yellow string 



