[ 3*3 } 
XVI. New Experiments on the Ocular Spe&ra of Light and 
Colours . By Robert Waring Darwin, M. D.; communicated 
by Erafmus Darwin, M. D. F . R. S. 
Read March 23, 1786. 
W HEN - any 1 one has long and attentively looked at % 
bright object, as at the fetting fun, on clofing Ms eyes* 
or removing them, an image, which refembles in form the 
objedt he was attending to, continues fome time to be vifible : 
this appearance in the eye we Mali calf the ocular fpedtrum of 
thatobjedh 
Thefe ocular fpedtra are of four kinds: ift, Such as- jare 
owing to a lefs fenfibility of a defined part of the retina ; jpr, 
fpedlra from defect of fenfibility* 2d, Such as are owing to a 
greater fenfibility of a defined part of the retina?; or fpedlra 
from excefs of fenfibility . 3d, Such as refemble their objeftMt 
its colour as well as form ; which may be termed diredl ocular 
fpedlra. 4th, Such as are of a colour contrary to that of their 
objedt; which may be termed reverfe ocular fipedtr a. ; ; 
- The laws of light have been mod: Tuccefsfully explained by; 
the great Newton, and the perception of vifible objedls has been 
ably inveftigated by the ingeniousDr. Berkeley and M.Male- 
branche ; but thefe minute phenomena of vifion have yet been 
thought reducible to no theory, though many philofopherk have 
employed a confiderable degree of attention upon them : among 
thefe are Dr, Jurin, at the end of Dr. Smith’s Optics ; JVh 
V ol» LXXVI. T t rf'EpiNus, 
