SECT. XL. OCULAR SPECTRA. 443 



SECT. XL. 



On the OCULAR SPFCTRA of Light and Colours, by Dr. R. W. 

 Darwin of Shrewfbury. Reprinted, by permiflion, from the 

 Philofophical Tranfadions, Vol. LXXVI. p. 313. 



Speclra of four kinds. I. Activity of the retina in vifton. 2. Spec- 

 tra from dejecl of fenftbility* 3. Speclra from excefs of fenfibili- 

 ty. 4. Of direcl ocular fpeclra. 5. Greater Jliumulus excites, 

 the retina into fpafmodic aclion. 6. Q/ reverfe occuiar fpeclra. 

 7. Greater Jlimulus excites the retina into various fuccejfive fpaf- 

 modic aftions. 8. Into fixed fpafmodic aclion. 9. Into tempora- 

 ry paralyKs. 10. Mifcellaneous remarks ; i. Dire 51 and re- 

 <u erf e fpeclra at the fame time. A fpeclra halo. Rule to prede- 

 termine the colours of fpeclra 2 Variation of fpeclra from ex- 

 traneous light. 3. Variation of fpeclra in number > figure^ and 

 remijjion. 4. Circulation of the blood in the eye is vifible. j. A 

 new way of magnifying objects. Conclujion. 



WHEN any one has long and attentively looked at a bright 

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

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

 tend, ng to, continues fome time to be vifible j this appearance 

 in the eye we (hall call the ocular fpeclrum of that object, 



Thefe ocular fpe^tra are of four kinds: ift, Such as are 

 owin^ to a kfs fenfibility of a defined part of the retina ; or 

 fpeftra from defecl of fenfibUity. ad, Such as are owing to a 

 greater fenfibility of a defined part of the retina ; or fpeclra from 

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

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

 tm 4th, Such as are of a colour contrary to that of their objedt; 

 which may be termed reverfe ocular fpeclra. 



The laws of light have been molt fuccefsfully explained by 

 the great Nekton, and the perception of vifible objects has been 

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

 branche ; but thefe minute phenomena of vifion have yet been 

 thought reducible to no theory, though many philofophers have 

 employed a confiderable degree of attention upon them : among 

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

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

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

 de T/Vcad des Scienc. 1765 ; M. de la Hire ; and, laftly, the 

 celebrated M. de BufFon, in the Memoires de 1'Acad. des Scien. 



who 



