170 ANNUAL OF SCIENTIFIC DISCOVERY. 



the camera lucida, or any other instrument known to him ; that a few trials 

 will be found sufficient to enable a person to use the instrument ; and that it 

 has the advantage of being portable, easily made, not difficult to use, and not 

 expensive. 



ON THE PHYSIOLOGY OF SIGHT. 



The following is an abstract of a communication presented to the American 

 Association, Providence meeting, by Dr. T. C. Hilgard : He commenced by 

 alluding to the faculty which the eye has of accommodating itself to different 

 distances of adjusting its focus. This consists not in any rounding out of 

 the cornea for near objects, or by any motion of the lens, as has been laid 

 down, but, as Dr. G-raefe has lately proved, in a lateral compression of the 

 eyeball, so as to give it greater depth. It has to be drawn out for near ob- 

 jects just as any other spyglass is. If any body will hold this print as closely 

 to his eye as possible, and make it distinct, he will feel a constriction of the 

 eyeball. The scope of sight, therefore, depends on the refractive power of 

 the cornea and the lens, and the casual shape of the bulb. Distinctness and 

 indistinctness of vision depend on a certain anatomical organization of the re- 

 tina, by which images under a certain size are not perceived ; on the power 

 of the center of the retina to distinguish forms more keenly than the outer 

 portions ; upon whether the retina is in focus or not ; upon the Y-formed ar- 

 rangement of the fibers of the lens, which often makes one see stars when he 

 does not bump his head ; upon the width of the pupil as the lateral part of 

 the lens do not unite rays into an exact focus ; upon the meeting of the axes 

 of the two eyes, for when both eyes are looking at one point, all objects, 

 farther or nearer, appear double, as they are not cast on the corresponding 

 places of the two eyes ; and upon the condition of the light arriving, which 

 is so serene on Alpine heights that black letters 1,500 feet high, set up against 

 snow-fields 40 miles distant, might be as easily read as letters one-twentieth 

 of an inch in height at the distance of 8 inches, the angle being the same. 

 "We have therefore a scope of distinct sight equal, within all distances to which 

 the eye can accommodate itself, for objects subtending the same angle. "We 

 have one limit of most minute vision, namely, as near as an object can be 

 borne, and also a comfortable distance for long-continued minute vision, such 

 as reading. Physiologists are still wondering why we see things upright, 

 when the image on the retina is upside-down, just as hi a camera-obscura. 

 The truth is that we do not consciously perceive the image ; we only perceive 

 by it, and our ideas of position are formed by the sense of touch. The new- 

 born infant has no clear vision ; at most it perceives only light and darkness ; 

 but it feels, and feeling is its only available sense ; it first becomes aware of 

 the position and form of things by feeling. And so, when we come to see, we 

 think that sight goes out and touches the objects that we see, and we say 

 with all the rest of the world, " As far as the eye can reach.' 1 ' 1 The fiery rings 

 which are seen when the eyes are pressed, although formed in the eye, seem 

 to be several inches before it. Persons who are born blind acquire a most 

 perfect knowledge of position by touch, and on receiving sight, immediately 

 perceive things upright. So, standing on the head alters nothing in our ideas 



