SECT. XL. 2. OCULAR SPECTRA. 609 



has fhewn that the cryftallinc humour itfelf confifts 

 of fibres* (Arcana IS at urse, V. i. p. 70.) 



Hence it appears, that as the mufcles have larger 

 fibres intermixed with a larger quantity of nervous 

 medulla, the organ of vifion has a greater quantity 

 of nervous medulla intermixed with fmaller fibres ; 

 and it is probable that the locomotive mufcles, as 

 well as the vafcular ones, of microfcopic animals 

 have much greater tenuity than thcfe of the retina. 



And bcfides the iimilarlaws, which will be fhewn 

 in this paper to govern alike the actions of the re- 

 tina and of the mufcles, there are many other ana- 

 logies which exift between them. 1 hey are both 

 originally excited into action by irritations, both 

 aft nearly in the fame quantity of time, are alike 

 ftrengthened or fatigued by exertion, are alike pain- 

 ful if excited into action when they are in an in- 

 flamed ftate, are alike liable to paralyiis, and to the 

 torpor of old age. 



II, OF SPECTRA FROM DEFECT OF SENSIBILITY. 



The retina is not fo eafily excited into aft'wn by lefs irri- 

 tation after having been lately fttbje&ed to greater. 



I. WHEN any one paffes from the bright daylight 

 into a darkened room, the irifes of his eyes expand 

 themfehes to their utmoft extent in a few feconds of 

 time; but it is very long before the optic nerve,, 

 after having been ftimulated by the greater light of 

 the day, becomes fenfible of the lefs degree of it in 

 the room ; and, if the room is not too obfcure, the 

 irifes will again contract themfelvcs in foiue degree, 

 as the fenfibility of the retina returns. 



2. Place about half an inch fquare of white paper 

 on a black hat, and looking fteadily on the center 

 of it for a minute, remove your eyes to a fheet of 



white 



