62 



SPECIAL SENSES. 



Fig. 39, 



which the rays must diverge, if a clearly defined image is to 

 be formed, is called the point of intersection, or focal centre.* 

 The position of this point is determinable, with the assistance of 

 an instrument for measuring angles ; it lies somewhat behind 

 the crystalline lens, and very near the centre of the eye. The 

 intersecting axal rays of two objective points (fig. 38) inclose 

 an angle (a, h, b, for the object a, b; i, k, h, for the object 

 i, k), which is called the visual angle. This angle diminishes 

 with the distance of the two objects from the eye, and the 

 retinal image is in the same proportion smaller. The arrow, 

 i, k, is only half the distance of the arrow, a, b, from the eye ; 

 the visual angle, i, h, k, is therefore twice as large as the angle 

 a, h, b, and the same thing is true in reference to the images 

 depicted upon the retina. It is on this account that objects of 

 different magnitudes seen at different distances, but of which the 

 visual angles are the same, form retinal images of the same size. 

 [ 132. All images falling upon the retina through the 



houses, &c., are perceived, forming a very elegant little picture, but re- 

 versed or upside down upon the posterior wall 

 of the transparent eye. If a simple or double 

 glass lens be now placed at a proper distance, 

 the reversed image which the objects refracted 

 by the crystalline lens form, may be projected 

 on a sheet of paper. 



* [Volkmann instituted many very able 

 experiments upon the condition of retinal 

 images, and from this inferred the focal centre. 

 An experiment easily performed is the fol- 

 lowing : Upon an horizontal table let a num- 

 ber of straight lines (fig. 39) a a', b b', &c., 

 he drawn, all of which intersect at the point 

 c; upon this point, c, let a prepared white 

 rabbit's eye, E, Y, E, be so placed, that the 

 axis of the eye coincides with the line a 7 , d 1 . 

 If the anterior part of the cornea, Y. stand at 

 the due distance from c, then will objects at 

 a, &, d, e, /, form their appropriate retinal 

 images at a", 6", d", e," /". The chamber 

 being darkened, let tapers be placed at , 6, 

 d, e, /, and the spectator look successively at 

 a, from a', at 6, from b', at d, from d\ cVc., 

 and it will be found that the line of vision 

 will cut the retinal image of a, at a", of b, at 

 &", &c. The retinal images of the whole of 

 the tapers lie in straight lines, which intersect 

 at the focal point, c. 



