J8 MOTIONS OF THE RETINA. SECT. III. j. 



infpefted, as explained in the experiments on ocular 

 fpeftra at the end of the work, and in Botanical 

 Garden, P. I. additional nore, No. I. Hence it ap- 

 pears, that a part of the retina, which had been fa- 

 tigued by contraction in one direction, relieves it- 

 ielf by exerting the antagonift fibres, and producing 

 a contraction in an oppofite direction, as is common 

 in the exertions of our mufcles. Thus when we are 

 tired with long action of our arms in one direction, 

 as in holding a bridle on a journey, we occafion- 

 ally throw them into an oppofite pofition to relieve 

 the fatigued mufcles. 



Mr. Locke has defined an idea to be " whatever 

 is prefent to the mind ;" but this would include the 

 exertions of volition, and the fenfations of pleafure 

 and pain, as well as thofe operations of our fyftem, 

 which acquaint us with external objects ; and is 

 therefore too unlimited for our purpofe. Mr. Locke 

 feems to have fallen into a further error, by con- 

 ceiving, that the mind could form a general or ab- 

 ftract idea by its own operation, which was the 

 copy of no particular perception ; as of a triangle 

 in general, that was neither acute, obtufe, nor 

 right angled. The ingenious Dr. Berkley and Mr. 

 Hume have demonftrated, that fuch general ideas 

 have no exigence in nature, not even in the mind 

 of their celebrated inventor. We ihall therefore 

 take for granted at prefent, that our recollection or 

 imagination of external objects confifts of a partial 

 repetition of the perceptions, which were excited 

 by thofe external objects, at the time we became ac- 

 quainted with them ; and that our reflex ideas of 

 the operations of our minds are partial repetitions 

 of thofe operations. 



II, The following article evinces that the organ 

 of vifion confifts of a fibrous part as well as of the 

 nervous medulla, like other white mufcles ; and 

 hence, as it refembles the mufcular parts of the 



body 



