THE RECENT PROGRESS OF THE 

 THEORY OF VISION. 



A COURSE OP LECTURES DELIVERED IN FRANKFORT AND HEIDEL- 

 BERG, AND REPUBLISHED IN THE PREUSSISCHE JAHRBTJCHER, 1868. 



I. THE EYE AS AN OPTICAL INSTRUMENT. 



THE physiology of the senses is a border land in which 

 the two great divisions of human knowledge, natural and 

 mental science, encroach on one another's domain ; in 

 which problems arise which are important for both, and 

 which only the combined labour of both can solve. 



No doubt the first concern of physiology is only with 

 material changes in material organs, and that of the 

 special physiology of the senses is with the nerves and 

 their sensations, so far as these are excitations of the 

 nerves. But, in the course of investigation into the 

 functions of the organs of the senses, science cannot avoid 

 also considering the apprehension of external objects, 

 which is the result of these excitations of the nerves, 

 and for the simple reason that the fact of a particular 

 state of mental apprehension often reveals to us a nervous 

 excitation which would otherwise have escaped our notice. 

 On the other hand, apprehension of external objects must 

 always be an act of our power of realization, and must 

 therefore be accompanied by consciousness, for it is a 

 mental function. Indeed the further exact investigation 

 of this process has been pushed, the more it has revealed 

 to us an ever-widening field of such mental functions. 



