466 OCULAR SPECTRA. SECT. XL. 10. 6. 



clofing my eyes, and (hading them fomewhat with my hand, 

 the word was di(lintly feen in the fpetrum in yellow letters 

 on a blue field ; and then, on opening my eyes on a yellowifh 

 wall at twenty feet diftance, the magnified name of BANKS 

 appeared written on the wall in golden characters. 



6. Conclufion. 



IT was obferved by the learned M. Sauvages (Nofol. Method. 

 Cl. VIII. Ord. i.) that the pulfations of the optic artery might 

 be perceived by looking attentively on a white wall well illumin- 

 ated. A kind of net-work, darker than the other parts of the 

 wall, appears and vanifhes alternately with every pulfation. 

 This change of the colour of the wall he well afcribes to the 

 cornpreflion of the retina by the diaftole of the artery. The va- 

 rious colours produced in the eye by the preflure of the finger, 

 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 tranfmitted by the laminae 

 of foap bubbles, or of air, according to their different thicknefs 

 or thinnefs, is it not probable, that the effedt 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 colours which ftimulate it into 

 adtion ? May not mufcular fibres exift in the retina for this pur- 

 pofe, which may be lefs minute than the locomotive mufcles of 

 microfcopic animals ? May not thefe mufcular ations of the ret- 

 ina conftitute the fenfation of light and colours ; and the volun- 

 tary repetitions of them, when the objet is withdrawn, confti- 

 tute our memory to them ? And laftly, may not the laws of the 

 fenfations of light, here inveftigated, be applicable to all our oth- 

 er fenfes, and much contribute to elucidate many phenomena of 

 animal bodies both in their healthy and difeafed ftate ; and thus 

 render this inveftigation well worthy the attention of the phyfi 

 cian, the metaphyfician, and the natural philofopher ? 



November i, 1785. 



Dum, Liber ! aftra petis volitans trepidantibus alis, 



Irruis immemori, parvula gijtta, mari. 

 Me quoque, mecurrente rota revolubilis aetas 



Volverit in tenebras, i, Liber, ipfe fequor. 



END OF THE FIRST PART; 



