SECT. XL. io OCULAR SPECTRA. 639 



appears and vaniflies alternately with every pupati- 

 on. This change of the colour of the wall he well 

 afcribes to the cotnpreffionof the retina by the diailole 

 of the artery. The various colours produced in the 

 eye by the preflure of the ringer, or by a ftroke on 

 it, as mentioned by Sir Ifaac Newton, feem likewife 

 to originate from the unequal preflure on various 

 parts of the retina. Now as Sir Ifaac Newton has 

 fhewn, that all the different colours are reflected or 

 tranfmitred by the laminse of foap bubbles, or of 

 air, according to their different thicknefs or thinnefs, 

 is it not probable, that the effect of the activity of 

 the retina may be to alter its thicknefs or thinnefs, 

 fo as better to adapt it to reflect or tranfmit the co- 

 lours which ftimulate it into action ? May not muf- 

 cular fibres exift in the retina for this purpofe, which 

 may be lefs minute than the locomotive mufcles of 

 microfcopic animals ? May not thefe mufcular acti- 

 ons of the retina conflitute the fenfation of light 

 and colours ; and the voluntary repetitions of them, 

 when the object is withdrawn, conftitute our memo- 

 ry of them ? And laftly, may not the laws of the 

 fenfations of light, here inveftigated, be applicable to 

 all our other fenfes, and much contribute to elucidate 

 many phenomena of animal bodies both in their 

 healthy and difeafed ftate ; and thus render this in- 

 veftigation well worthy the attention of the phyfician, 

 the nietaphyfician, and the natural philoibpher ? 



November i, 1785. 



ADDITION. 



