EYES USUALLY UNEQUAL. 



sliding interior box B' c' be moved until, on closing the left eye, 

 the division of the scale coincides with the edge M of the open- 

 Fig. 5. 



ing, and at the same time, by closing the right eye, the same 

 division of the scale coincides with the edge N of the opening. 

 It will be always possible to make this adjustment, provided the 

 eyes are placed centrally opposite the opening M N, which may be 

 easily managed by cutting in the edge of the box A D an opening 

 to receive the bridge of the nose. This arrangement being made, 

 it is clear that if we close the left eye we shall see the space 

 upon the scale included by the lines E, N and E if continued 

 to the screen R' if. Let us suppose this space to include the six 

 divisions of the scale from to 6. If we close the right eye, 

 we shall see with the left eye the six divisions' of the scale to 

 the right of 0. Now if we open both eyes and look steadily with 

 them through the aperture M N, giving no more attention to the 

 impression on the one than on the other, we shall see the twelve 

 divisions of the scale, six to the right and six to the left of ; 

 the six divisions to the left of being seen only with the right 

 eye, and the six divisions to the right of being seen only with 

 the left eye. 



In this way we have two similar objects, similarly illuminated 

 and of equal magnitude, in immediate juxtaposition, the one seen 

 by the right and the other by the left eye ; and any difference in 

 their distinctness, quality, brilliancy, or colour, will be as clearly 

 and instantly perceivable as the comparative brilliancy of spaces 

 illuminated by two different lights in the photometer. I have 

 already experimented with this apparatus upon my own eyes, the 

 result of which is, that I find that the sight of the right eye is 

 much better than that of the left, the figures to the left of being 

 always more distinct than those to the right of it : but, what is 

 more remarkable, I find that the transparency of the humours of 



203 



