IN OPTICS. 91 



impressions of the same strength as the one to 

 which we might desire particularly to attend ; 

 whereas, in its present state, the vivacity of the 

 impression from the object, to which we turn 

 the optic axis, most commonly surpasses, con- 

 siderably, that of every other upon the same 

 membrane ; by which means our attention is 

 rendered less liable to interruption. 



Secondly ; The extension of accurate vision, 

 to every part of the retina, would deprive us, in 

 great measure, of the help, which we obtain, at 

 present, from the eye, in learning the thoughts 

 of other men. As far as I have been able to 

 observe, the changes produced by our internal 

 feelings, upon the state of the eye itself* are 

 very few, and relate only to the quantity of 

 moisture, which is diffused over its surface, and 

 the degree of fulness in the blood-vessels, which 

 are spread upon its white and glistening part. 

 Both of these circumstances, however, are 

 similarly altered by opposite passions, and, con- 

 sequently, neither of them can be regarded as 

 the appropriate expression of any. The whole 

 variety, then, of the expressions of feeling which 

 are justly attributed to the eye, must, I think, 

 depend upon its motions. Some of these are 

 the immediate effects of certain passions ; the 

 eye, for instance, being moved differently in 

 anger and in grief 5 and such may be esteemed 



