SIGHT. 



seat of vision. But Mariotte, having found that images which fall 

 on that part of the retina where the optic nerve enters it are 

 invisible, concluded, because where there is no chorioid there is 

 no vision, that the chorioid performs the functions attributed by 

 most physiologists to the retina. 



Mariotte performed his well-known experiment in the following 

 manner : He made two spots in the same horizontal line upon the 

 wall of a room, and, having closed the left eye, placed the right 

 eye opposite the left spot, and gradually moved backwards until the 

 right hand spot disappeared, a circumstance which occurs when 

 the image falls on the place where the optic nerve enters the retina. 

 The experiment succeeds very well when two wafers are placed 

 on the wall about three inches apart, and the observer commences 

 to recede when at the distance of twelve inches from them. A 

 very ready mode of trying this experiment is the following : 

 Place two coloured wafers upon a sheet of white paper, close 

 one eye and fix the other opposite one of the wafers, then move 

 the outside wafer, always in the same straight line, until it is 

 invisible ; when in this situation, if the wafer be moved to the 

 right, to the left, above, or beneath, it is again seen. 



Purkinje has remarked that the spot of the retina is not insensi- 

 ble to the stimulus of light, as it is generally stated to be ; for, if a 

 candle be substituted for the moving wafer of the preceding expe- 

 riment, though the flame is not seen, a red glare is perceived. It can 

 scarcely be doubted that the insensible point is the place occu- 

 pied by the central artery of the retina, and the chief argument 

 for the chorioid being the seat of vision therefore falls to the 

 ground. 



We are indebted to Purkinje for a most beautiful experiment, 

 by which a person may see the blood-vessels of his own retina. 

 The experiment succeeds best in a dark room, when one eye is 

 shaded from the light, and the flame of a candle is placed by the 

 side of the unshaded eye, but so as not to occupy any of the 

 central part of the field of view. So long as the flame of the 

 candle remains stationary, nothing further occurs than a diminu- 

 tion of the sensibility of the retina to light : but, after the flame 

 has been moved upwards and downwards, through a small space, 

 for a length of time, varying with the susceptibility of the person 

 on whom the experiment is tried, the phenomenon presents itself. 

 The blood-vessels of the retina, exactly as represented in the 



