HOW W'V. SI'l:. 



Lecture delivere.l in the United States Nationcl Muscnni, April 2<t. 1.S.S2, liy 

 Dr. Swan M. Burneti-. 



LaDIKS and (lENTLK.NfKN: 



WIhmi man Hrst IuuikI liiiusclf capable of forming a proper 

 judiiinoiit of the wurkinjj^s of Nature, the sense of vision 

 must luive excited in his mind an emotion of the greatest 

 wonder. That he was co<;-ni/,ant of o]>jects situated at dis- 

 tances very remote and hcvond tiic reach of his touch, and 

 throu^li a medium the very exislence-of which was a mys- 

 tery al)()ve his compreiiension, must liavc seemed to him a 

 prol)leni whicli <Ietied even an attempt at sohition. We are 

 not aware that any effort lias been made by savage or har- 

 l>arous tribes to account for any of the phenomena of sight. 

 They give I'cady explanations of the cause of thundei- and 

 lightning, of the origin of men and animals, the creation 

 of hre and other natural i)henomena, but the sense of sight 

 is so wholly unlike anything else with which they are 

 familiar that they have no analogies t.o fall back on, and 

 must accept it as a jaimary gift of the Divine Spirit. It is 

 only when the intellect has developed to such a degree as to 

 enable it to consider things as related to each other, and 

 analyse phenomena as they ])resent them.selvcs to the mintl, 

 reducing them as far as possible to their constituent ele- 

 ments, that the senseof vision is considered a subject within 

 the scope of human investigation. The i)hysiology of vision, 

 therefore, belongs preeminently to the .scientific era in the 

 hi.^tory of intelleitnal development. It lind-^ no |tlace, so far 

 as I know, in the era of superstition. T(. tell you what the 

 scientific; method of thought has accomplished in this field 

 is the object of the remarks we shall make this afternoon. 



In the study of the phenomena jMcsented by the sense of 

 vision we find four separate links in a chain of .sequences. 

 If any one of these links is missing there can be no .sensa- 

 tion of sight. 



The fir^t link in the chain, naturally, is the object to be 

 seen; the second, the medium connecting this object with 



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