Sect. XL. OCULAR SPECTRA, 44* 



SECT. XL. 



On the Ocular Spectra of Light and Colours, by Dr. R. W. 

 Darwin, of Shrewfbury. Reprinted, by permiflion, from the 

 Philofophical Tranfactions, Vol. LXXVI. p. 313. 



Spectra of four kinds. 1. Activity of the retina in vifion. 2. Spec- 

 tra from defect of fenftbility. 3. Spectra from excefs of fenfibili- 

 ty. 4. Of direct ocular fpectra. 5. Greater fiimulus excites 

 the retina into fpafmodic action. 6. Of reverfe ocular fpectra. 

 7. Greater flimulus excites the retina into various fuccejfve fpaf- 

 modic actions. 8. Into fixed fpafmodic action. 9. Into tempora- 

 ry paralyfts. I o. JMifcellaneous remarks ; 1 . Direct and re- 

 verfe fpectra at the fame time. A fpectral halo. Rule to prede- 

 termine the colours of fpectra. 2. Variation of fpectra from ex- 

 traneous light. 3. Variation of fpectra in number, figure, and 

 remiffion. 4. Circulation of the blood in the eye is vifibie. 5. A 

 new way of magnifying objects. Conclufion. 



When any one has long and attentively looked at a bright 

 object, as at the fetting fun, on clofing his eyes, or removing 

 them, an image, which refembles in form the object he was at- 

 tending to, continues fome time to be vifibie ; this appearance 

 in the eye we (hall call the ocular fpectrum of that object. 



Thefe ocular fpectra are of four kinds : ift, Such as are 

 owing to a lefs fenfibility of a defined part of the retina ; or 

 fpectra from defect of fenfibility. 2d, Such as are owing to a 

 greater fenfibility of a defined part of the retina \ ox fpectra from 

 excefs of fenfibility. 3d, Such as refemble their object in its 

 colour as well as form ; which may be termed direct ocular fpec- 

 tra. 4th, Such as are of a colour contrary to that of their object ; 

 which may be termed reverfe ocular fpectra. 



The laws of light have been moil fuccefsfully explained by 

 the great Newton, and the perception of vifibie objects has been 

 ably inveftigated by the ingenious Dr. Berkeley and M. Male- 

 branche ; but thefe minute phenomena of vifion have yet been 

 thought reducible to no theory, though many philolbphers have 

 employed a confiderable degree of attention upon them : among 

 thefe are Dr. Jurin, at the end of Dr. Smith's Optics ; M. 

 iEpinus, in the Nov. Com. Petropol. V. 10.; M. Beguelin, in 

 the Berlin Memoires, V. II. 1771 ; M. d'Arcy, in the Hiitoire 

 de l'Acad. des Scienc. 1765 •, M. de la Hire; and, laftly, the 

 celebrated M. de BufFon, in the Memoires de l'Acad. des Scien. 



who 



