446 OCULAR SPECTRA. SECT. XL. 2. i. 



when they are in an inflamed ftate, are alike liable to paralyfis, 

 and to the torpor of old age. 



II. OF SPECTRA FROM DEFECT OF SENSIBILITY. 



The retina is not fo eafily excited into aElion by lefs irritation after 

 having been lately fubjeEted to greater. 



1 . WHEN any one pafTes from the bright day-light into a dark- 

 ened room, the irifes of his eyes expand themfelves to their ut- 

 moft extent in a few feconds of time ; but it is very long before 

 the optic nerve, after having been itimuiated by the greater light 

 of the day, becomes fenfible of the lefs degree of it in the room ; 

 and if the room is not too obfcure, the irifes will again contrail 

 themfelves in fome degree, as the fenfibility of the retina returns. 



2. Place about half an inch fquare of white paper on a black 

 hat, and looking fteadily on the centre of it for a minute, remove 

 yoar eyes to a (heet of white paper ; and after a fecond or two 

 a dark fquare will be feen on the white paper, which will con- 

 tinue fome time. A fimilar dark fquare will be feen in the 

 clofed eye, if light be admitted through the eyelids. 



So after looking at any luminous object of a fmall fize, as at 

 the fun for a (hort time, fo as not much to fatigue the eyes, 

 this part of the retina becomes lefs fenfibie to fmaller (Quantities 

 of light ; hence, when the eyes are turned on other lefs lumi- 

 nous parts of *he iky, a dark fpot is feen refembling the fhape of 

 the fun, or other luminous objeft which we lair, beheld. This 

 is the four'ce of one kind of the dark-coloured mufca volitantes. 

 If this dark fpot lies above the centre of the eye, we turn our eyes 

 that way c xpefting to bring it into the centre of the eye, that we 

 may view it more diftindlly ; and in this cafe the dark fpetlrum 

 feems to move upwards. If the dark fpectrum is found beneath 

 the centre of the eye, we purfue it from the fame motive, and it 

 fcems to move downwards. This has given rife to various con- 

 jectures of fomething floating in the aqueous humours of the 

 e^s ; but whoever, in attending to thefe fpots, keeps his 

 eyes unmoved by looking fteadily at the corner of a cloud, at 

 the fame time that he obferves the dark ipeftra, will be thor- 

 oughly convinced, that they have no motion but what is given 

 to them by the movement of our eyes in purfuit of them. Some- 

 times the form of the fpedtrum, when it has been received from 

 a circular luminous body, will become oblong ; and fometinies 

 it will be divided into two circular fpeftra, which is not owing 

 to our changing the angle made by the two optic axifes, accord- 

 ing to the inftance of the clouds or other bodies to which the 



fpe&rum 



