354 Mr. Shelford Bidwell [March 5, 



WEEKLY EVENING MEETING, 



Friday, March 5, 1897. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.R.S. Honorary 

 Secretary and Vice-President, in the Chair. 



Shelford Bidwell, Esq. M.A. LL.B. F.R.S. M.R.L 



Some Curiosities of Vision. 



The function of the eye, regarded as an optical instrument, is limited 

 to the formation of luminous images upon the retina. From a purely 

 physical point of view it is a simple enough piece of apparatus, and, 

 as was forcibly pointed out by Helmholtz, it is subject to a number of 

 defects which can be demonstrated by the simplest tests, and which 

 would, in a shop-bought instrument, be considered intolerable. 



What takes jjlace in the retina itself under luminous excitation, 

 and how the sensation of sight is produced, are questions which 

 belong to the sciences of physiology and psychology ; and in the 

 physiological and psychological departments of the visual machinery 

 we meet with an additional host of objectionable peculiarities from 

 which any humanly constructed apparatus is by the nature of the case 

 free. 



Yet in spite of all these drawbacks our eyes do us excellent 

 service, and provided that they are free from actual malformation and 

 have not suffered from injmy or disease, we do not often find fault 

 with them. This, however, is not because they are as good as they 

 might be, but because with incessant practice we have acquired a very 

 high (iegree of skill in their use. If anything is more remarkable 

 than the ease and certainty with which we have learnt to interpret 

 ocular indications when they are in some sort of conformity with ex- 

 ternal objects, it is the pertinacity with which we refuse to be misled 

 when our eyes are doing their best to deceive us. In our earliest 

 years we began to find out that we must not believe all we saw : 

 experience gradually taught us that on certain points and under 

 certain circumstances the indications of our organs of vision were 

 uniformly meaningless or fallacious, and we soon discovered that it 

 would save us trouble and add to the comfort of life if we cultivated 

 a habit of completely ignoring all such visual sensations as were of 

 no practical value. In this most of us have been remarkably suc- 

 cessful, so much so that, if from motives of curiosity or for the sake 

 of scientific experiment, we wish to direct our attention to the sensa- 

 tions in question and to see things as they actually appear, we can 



