OF SIGHT. 181 



and gradually to retire, till the right spot was no longer distin- 

 guishable. This is said to occur when its image falls upon the 

 centre of the optic nerve. I should suppose that it disappeared 

 at a certain distance, merely because too minute for detection 

 when the eye was being directed to the other spot. For the 

 same reason, in Picard's variation of this experiment, by placing 

 an object between the eye and the spots, so that it shall appear 

 double and one image of it cover one spot completely when one 

 eye is closed, the disappearance of the spots at the time the in- 

 tervening object is looked at, probably arises from the one spot 

 being covered by it, as in truth only one eye is here employed 

 although both are open, and from the other spot being too dis- 

 tant to be visible when the attention is directed to the inter- 

 vening object. 



(E) The notion o'f our originally seeing objects upside-down, 

 double, and all as at the same distance, is satisfactorily refuted 

 by Dr. Spurzheim. The organs of sight, and all the others of sense, 

 present, if perfect, a perfect impression to the inward senses, 

 the faculties for judging of form, distance, colour, tune, &c. and 

 nothing farther. These do the rest. My reader must consult 

 Dr. Spurzheira, and particularly Mr. Coombe. 



{F) Although we certainly use both eyes to look generally at 

 objects before us (those on each side can of course be seen by 

 the eye of the same side only), yet when we fix attentively on 

 an object, we employ but one. This at least is my own case. 

 If I hold up a finger, and look at distant objects, it appears 

 double, and if I then look at it, I of course see it single, and the 

 figure now seen is, in my case, that which was previously seen 

 Avith the right eye : no difference occurs in it, if now the left eye 

 is closed.* The greater facility of threading a needle, when both 

 are open, probably arises from the advantage of increasing the field 

 of vision while one eye is fixed steadily upon the aperture. 



* See also Spurzheim. 1. c. p. 299. 



