68 4 THE POPULAR SCIENCE MONTHLY. 



that, no mitter under what conditions it may be compelled, by the 

 higher law of mutual repulsion, to live, it will mould its own organism 

 into harmony with those conditions, and thus continue its existence ; 

 and this, whether it is required to adopt* a more perfect or a less per- 

 fect form. 



But what it actually is, is no criterion of what it is capable of be- 

 coming, and the locality in which it is found is no evidence that it is 

 best adapted to such a locality. These data only prove that in the 

 final balance of forces to which it is subjected it was assigned such 

 a degree of development and such a habitat. 



OBSERVING THE INTERIOR OF THE EYE. 1 



Br JULIUS BERNSTEIN, ' 



PEOFE8SOE OF PHYSIOLOGY IN THE UNIVERSITY OF HALLE. 



THE retina is the point where the physical process of vision passes 

 into the physiological process. Until it impinges upon the retina, 

 the light which penetrates the eye has only undergone physical 

 changes, consisting chiefly in refraction, the last perceptible result of 

 which is the production of the image upon the retina. From this 

 point the process passes from our immediate observation, and the 

 difficulty of discovering its character increases at each step. The 

 image upon the retina is reversed, and yet we see every object in the 

 field of vision upright. This is the result of the experience, which we 

 have acquired from childhood, in the exercise of the organ of sight. 

 The point A (Fig. 1), which is on the right, is imprinted upon the left 

 portion of the retina, and we, therefore, know by experience that a 



Fig. i. 



ray, coming from the right, must strike the left portion of the retina ; 

 and because we always imagine the objects we see to be external to 

 ourselves, we must do so by unconsciously following the line a A, 

 through the optical centre Jc. In this manner the eye projects a uni- 

 form field of vision, which is obtained by drawing, from every point 

 of the retina outward, straight lines through the optical centre of the 

 eye, which lines will terminate upon a convex surface. 



This is really the manner in which the eye interprets, in all cases, 



1 From " The Five Senses of Man," No. XXI. of the " International Scientific Series." 



