6co OCULAR SPECTRA. SECT. XL. a, 



white paper ; and after a fecond or two a dark fquare 

 will be feen on the white paper, which will conti- 

 nue fome time. A iimilar 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 fliort time, fo as not much 

 to faiigue the eyes, this part of the retina becomes 

 lefs fen fib le to fmaller quantities of light ; hence, 

 when the eyes are turned on other lefs luminous 

 parts of the fky, a dark fpot is feen refembling the 

 fliape of the fun, or other luminous object which 

 we laft beheld. This is the fource of one kind of 

 the dark-coloured mufca vplitantes. If this dark fpot 

 lies above the cepter of tlfe eye, we turn our eyes 

 that way, expecting to bring it into the center of 

 the eye, that we may view it more distinctly ; and 

 in this cafe the dark fpectrum feems to move up- 

 wards. If the dark fpectrum is found beneath the 

 centre of the eye, we purfue it from the fame mo- 

 tive, and it fee ins to move downwards. This has 

 given rife to various conjectures of fomething float- 

 ing in the aqueous humours of the eyes ; but who- 

 ever, 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 

 fpectra, will be thoroughly convinced, that they 

 have no motion but what is given to them by the 

 movement of our eyes in purfuit of them. Si>me- 

 times the form of the fpectrurn, when it has been 

 received from a circular luminous body, will be- 

 come oblong ; and fometimes it will be divided into 

 two circular fpectra, which is not owing to our 

 changing the angle made by the two optic axifes, 

 according to the diftance of the clouds or other 

 bodies to which the fpectrum is fuppofed to be 

 contiguous, but to other caufes mentioned in ISo. 



Xo 

 J 



