io MOTIONS OF SECT. III. 2. r. 



vioufly infpeeted, as explained in the experiments on ocular 

 fpe&ra at the end of the work, and in Botanical Garden, P. L 

 additional note. No. I. Hence it appears, that a part of the ret- 

 ina, which had been fatigued by contraction in one direction, 

 relieves itfelf by exerting the antagonift fibres, and producing a 

 contraction in an oppofite direction, as is common in the exer- 

 tions of our mufcles. Thus when we are tired with long ac- 

 tion of our arms in one direction, as in holding a bridle on a 

 journey, we occafionally 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 opera- 

 tions 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 conceiving, 

 that the mind could form a general or abftract 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 exiftence in na- 

 ture, not even in the mind of their celebrated inventor We 

 fhall 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 ob- 

 jects, at the time we became acquainted with them j and that 

 our reflex ideas of the operations of our minds are partial repe- 

 titions of thofe operations. 



II. The following article evinces that the organ of vifion con- 

 fifts of a fibrous part as well as of the nervous medulla, like oth- 

 er white mufcles \ and hence, as it refembles the mufcular parts 

 of the body in its ftrufture, we may conclude, that it muft re- 

 femble them in pofTefTing a power of being excited into animal 

 motion. The fubfequent experiments on the optic nerve, and 

 on the colours remaining in the eye, are copied from a paper on 

 ocular fpectra, publifhed in the feventy-fixth volume of the Phi- 

 lof Trani". by Dr. R. Darwin of Shrewfbury ; which, as I ihall 

 have frequent occafion to refer to, is printed in this work, 

 Sect. XL. The retina of an- ox's eye was fufpended in a glafs 

 of warm water, and forcibly torn in a few places ; the edges of 

 thefe parts appeared jagged and hairy, and did not contract and 

 become fmooth like fimple mucus, when it is diilended till it 

 breaks, which evinced that it confided of fibres. This fibrous . 

 conftruction became (till more diftinct to the fight by adding 

 forne cuuftic alkali to the water : as the adhering mucus was 



firft 



