[ z6 + ] 
or defedt in either of thefe making a very fenfible 
difference in the colour. 
It will now naturally be afked, why the frequency 
of the changes of brightnefs fhould not be often much 
greater, as well as fometimes lefs, than that above- 
mentioned, and why the interval of the fourth or fifth, 
or fome fuch part, fhould be pitched upon, rather than 
the fortieth or fiftieth part of a fecond, or than a whole 
fecond, &c. for, according to the length or fiiortnefs of 
the time affumed, the changes, that will naturally 
occur, from the effedt of chance, will be fmaller or 
greater in proportion to each other. The anfwer to this 
queflion will, I think, tend to render the above folution 
more probable, as well as to throw a good deal of 1 ght 
upon the whole fubjedt. The lengths of the times then 
between the changes of brightnefs, if I am not mif- 
taken, depend upon the duration of the perception 
before mentioned, occafioned by the imprefiion of 
the light upon the eye, than which they feem to be 
neither much longer nor fhorter. Whatever inequa- 
lities fall within a much fhorter time than the conti- 
nuance of this perception, will neceflarily be blended 
together, and have no effect, but as they compofe a 
part of the whole mafs ; but thofe inequalities, which 
fall in fuch a manner as that they may be affigned to 
intervals nearly equal to, or fomething greater than 
the continuance of this perception, will be fo divided 
by the imagination, which will naturally follow, and 
pick them out as they arife. 
END OF PART I. 
