6o6 OCULAR SPECTRA. SECT. XL, i. 



defined part of the retina ; or fpefira from excefs of 

 fallibility. 3d, Such as referable their object in its 

 colour as well as ioi m ; which may be termed direft 

 ocular ffe&ra. 4th, Such as are of. a colour con- 

 trary to that of their object ; which may be termed 

 rewrfe ocular fpeftra. 



The laws of light have been moft fuccefsfully ex- 

 plained by the great Newton, and the perception of 

 vifible objects has been ably investigated by the in- 

 genious Dr. Berkeley and M. Malebranche ; but 

 thefe minute phenomena ot vifion have yet been 

 thought reducible to no theory, though man) philo- 

 fophers have employed a conliderable degree of at- 

 tention upon them : among thefe are Dr. Jurin, at 

 the end of Dr. Smith's Optics; M. jEpinus, in the 

 Mov. Com. Petropol. V. 10, ; M. Beguelin, in the 

 Berlin Memoires, V. II 1771 , M. D'Arcy, in the 

 Hiftoire de 1'Acad. des Scienc. 1765 ; M. de la 

 Hire; and laflly, the celebrated M. de Button, in 

 the Memoires de 1'Acad des Seien. who has termed 

 them accidental colours, as if fubje&ed to no eftab- 

 Jiflied laws, Ac. Par. 1743. M p. 215. 



I muft here apprize the reader, that it is very dif- 

 ficult for different people to give the fame names to 

 various fhades of colouis ; whence, in the following 

 pages, fomething muft be allowed, if on repeating 

 the experiments the colours here mentioned mould 

 not accurately correfpond with his own names of 

 them. 



I. A&faity of the Retina in Vifion* 



FROM the fubfequent experiments it appears, that 

 the retina is in an aiive not in a paflive Hate during 

 the exiftence of thefe ocular fpe&ra; and it is thence 

 to be concluded, that all viiion is owing to the ac- 

 tivity of this organ. 



i. Place 



