?A8 Mr. T. Smith 07i certain Pkanomena of Vision. 



duced by a function of the brain will not, I believe, be dis- 

 puted, since injury of the brain by mechanical compression, &c. 

 destroys the function, and the contractions and dilatations 

 of the iris that are observed to accompany its exercise. In 

 this case, therefore, distinct vision is the end for which a cer- 

 tain function of the brain, as well as a certain mechanism of 

 the eye, is provided ; and the following considerations, sug- 

 gested by the effects on the objects produced by the affections 

 we are investigating, lead to similar conclusions in regard to 

 the end or purpose of them. It is well known that the vision 

 cannot be adapted in the common way to more than one di- 

 stance at once. Now when the eyes are adapted in this way 

 to objects at one distance, objects that are nearer or further 

 off than that distance, are actually made more distinct than 

 they would otherwise be by the operation of the two affections 

 we have been examining. By one of these the indistinctness 

 of a brighter object is lessened by the sensibility being in- 

 creased to the colour of the object itself, and diminished to 

 the same colour in less luminous objects around it, thus making 

 the principal object brighter and better defined by a double 

 contrast. By the other, the indistinctness arising from the 

 chromatic aberration is removed by insensibility to the red or 

 violet rays bordering the image; and as that insensibility ex- 

 tends over a wider space than the red or violet border occu- 

 pies, the false vision thus occasioned is corrected by the sen- 

 .sibility to red light in the other eye being increased, in exactly 

 the same degree as it is diminished in the exposed eye. Both 

 affections, therefore, have the characters of perfect functions 

 admirably contrived, it must be acknowledged, and as well 

 adapted to produce distinct vision as can well be imagined in 

 the circumstances. 



Having thus given, in as compressed a form as I could adopt 

 injustice to the subject, a full account of this investigation, I 

 forbear, for the present, from making any observations on the 

 singular nature of the cereh7-al J'lmctions thus detected, or on 

 the perhaps still more singular nature of their exciting causesy 

 thinking it due to truth, in a case that appears to involve prin- 

 ciples entirely new, to wait the observations of competent 

 inquirers, with whom it remains to confirm or refute, by an 

 impartial scrutiny, the results which I have obtained. I shall, 

 therefore, conclude with a short summary of those results. 



1st. Besides the well-known function by which the eyes are 

 adapted, by turns, to different distances, txsoo other Junctions, 

 hitherto unknown, are occasionally called to the aid of 

 vision. 



2nd. Both of these newly observed functions are excited by 



