466 OCULAR SPECTRA. Sect. XL. 10. 6. 



clofing my eyes, and fhading them fomewhat with my hand, 

 the word was diftinctly feen in the fpectrum 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. 

 CI. 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 vaniihes alternately with every pulfation. 

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

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

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

 or by a ftroke on it, as mentioned by Sir Ifaac Newton, feem 

 likewife to originate from the unequal prefTure on various parts 

 of the retina. Now as Sir Ifaac Newton has fhewn, that all 

 the different colours are reflected or tranfmitted 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 colours which ftimulate it into 

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

 pofe, which may be lefs minute than the locomotive mufclesof mi- 

 cro fcopic animals ? May not thefe mufcular actions of the ret- 

 ina constitute the fenfation of light and colours ;^ and the volun- 

 tary repetitions of them, when the object is withdrawn, confti- 

 tutc our memory of them ? And laitly, 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- 

 clan, the metaphyUcian, and the natural philofopher ? 



November i, 1785. 



Dum, Liber ! aftra petis volitans trepidantibus ali», 



Irruis immemori, parvula gutra, mari. 

 Me quoque, me currentc rota revolubilis :etas 

 lebrae, — i, Liber, ipfe fequor. 



) OF THE TJRST PART. 



