1896-97-] THK FUNCTION OF INDIRECT VISION. 305 



THE FUNCTION OF INDIRECT VISION AND THE USE 

 OF COLOURED AND SMOKED EYE GLASSES. 



By a. Kirschmann, Ph.D. 



(Read April 24th, 18^7). 



The proposition that the sensibility of the retina decreases from the 

 fovea centralis toward the periphery is still seen now and then in text 

 books on Physiological Optics. This statement shows not only a care- 

 less misrepresentation of facts which can be observed in our daily 

 experience, and an entire lack of understanding of the extremely 

 elaborate adaptation of this sense organ to the difficult function which it 

 has to perform, but it also involves a logical error, for those who make 

 such a statement either do not see that the retina has different func- 

 tions to accomplish, thus requiring a special sensibility in each of these 

 functions, or they assume that if the peripheral retina is less sensitive 

 for one quality of the impressions it must be less sensitive for all. 

 They also seem to assume that the conditions which prevail on the 

 eccentric retina are of the nature of an imperfection, or a less complete 

 development. 



The negligent way in which indirect vision has hitherto been treated, 

 even in the most celebrated works on Physiological and Psychological 

 Optics, has borne evil fruit of which I may here give a simple example. 

 In Germany they are about to replace the customary German type with 

 the Roman, on the ground that the latter has simpler forms and can 

 therefore be more easily distinguished. This contention contains the 

 silent assumption that all reading is done by means of direct vision, 

 that is, that all single letters are successively projected on the fovea 

 centralis. This, however, is not at all the case, for we do not let the 

 fixation point wander from letter to letter as a child does in its first 

 reader, but the fixation point jumps from word to word, fixing in each 

 a letter at random and seeing all others indirectly. (This circumstance 

 accounts also for the fact that we overlook mistakes in printing so 

 easily, especially when the subject matter is familiar to us). Thus we 

 see that reading does not require direct vision only, but it depends even 

 for the greater part on indirect vision. In fact, with indirect vision 

 alone with a little practice we could manage to read tolerably well, but 



