7 
9^ Mi* Smith on the Jinctuaiing Sensibility the Retina. 
the bank of the rivulet, we rested aftef our fatigues. We were 
^lad to be beyond the hoarse cries of the birds, and to leave a 
place where darkness does not offer even the charm of silence 
and tranquillity. We could scarcely persuade ourselves, that the 
name of the Grotto of Caripe had hitherto remained unknown 
in Europe. The giiacharoes alone would have been sufficient to 
render it celebrated* These nocturnal birds have been nowhere 
yet discovered, except in the mountains of Caripe and Cumana- 
coa. 
The cavern of Caripe, situated nearly in the latitude of 
10° 10', consequently in the centre of the torrid zone, is elevated 
606 toises above the level of the water in the Gulf of Cariaco. 
We found in every part of it, in the month of September^ the 
temperature of the interior air between 64°.6 and 66° of Fah- 
renheit ; the external atmosphere being at 61°.S* At the en- 
trance of the cavern, the thermometer in the air was at 63°.T ; 
but when immersed in the water of the little subterraneous river, 
it marked, even to the end of the cavern, 62°.2* 
Art. XVI . — On the fluctuating Sensibility of the Retina. By 
Mr Thomas Smith, Surgeon^ KinguisSie. Communicated 
by the Author. 
The phenomena of Accidental Colours have been attempted 
to be accounted for, on the principle that the sensibility of the 
tetina is diminished by the action of light. “ The eye,”” says 
the celebrated Cuvier, ‘‘ sometimes sees what are called Acci- 
dental Colours. When the retina is too much fatigued by 
certain colours, it becomes less sensible of them. If we then 
behold a colour which admits in its coniposition that which had 
fatigued the eye, the latter will cease to be visible. Thus, when 
we fix the eye on a white spot, and afterwards tUrn it to White 
bodies, we observe on them a dark spot of the same shape as 
that to which the eye was fifst directed. If the spot on which 
the eye was fixed was black, the eye, wherever it turns, will 
perceive one of a lighter coloUr. If it was red, we perceive a 
greenish spot oh white bodies ; if yellow, a bluish ; if green, a 
reddish,’’ &c. 
