PHYSICAL anp LITERARY. 77 
per be daubed all over with fmall dots of 
blue and yellow, it will appear green to 
an 
The total impreffion made upon the eye by equal fmall 
parts of the feétor mult be, as the quantity of Jight emit- 
ted from it and the frequency of the returns of the rod 
to it; i,¢. inyverfely, as the time between the returns of 
the rod. Let ABC reprefent the circular fector, and DC 
_ a line bifedting it; the rod always returns to DC after the 
time of one vibration; and, to.any other line EC 
between DC and AC or AB, the mean time of its 
return is the fame; for it alternately returns in twice 
the time of deicribing AE and twice the tine of de- 
{cribing EB; fo that two fucceeding intervals ot its 
returns are equal to the time of two vibrations: But the 
intervals of the returns to the lines AC or CB ate mani- 
feftly equal to the time of two entire vibrations. The 
brightnefs of the fector therefore in DC, ar any line be- 
tween DC and AB or BC, mutt be fimply as the quantity 
| ipbot light emitted from equal fmall portions of the fe@or ; 
that is, in the inverfe proportion of the velocities of the 
rod when in thefe lines. It is plain from this, that the 
- fetor mutt be ixcomparably brighter in AC and BC, where 
it refts, than any where elfe, notwithftanding that the 
intervals of return thither are double ; that is, it will 
appear to be bounded diftinatly with a white rod on each 
fide. 
If the rod be agitated with fmall and quick vibrations . 
of its own, by fticking it again{t fome folid body imme- 
diately before it is hurried backw ards and forwards with 
the 
