SOME CURIOSITIES OF VISION. 1 



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



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 place 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 injury 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 

 degree of skill in their use. If anything is more remarkable than the 

 ease and certainty with which we have learned to interpret ocular indi- 

 cations when they are in some sort of conformity with external 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 fal- 

 lacious, 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 successful, so much so that, if from motives 

 of curiosity or for the sake of scientific experiment, we wish to direct 

 our attention to the sensations in question and to see things as they 



'From Proceedings of the Royal Institution of Great Britain, Vol. XV, Part II, 

 No. 91, April, 1898, pp. 354-365. Read at weekly evening meeting, Friday, March 

 5, 1897. 



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