70 ESSAYS AND OBSERVATIONS 



paper be daubed all over with fmall dots of 

 jbluc and yellow, it will appear green to an eye 

 which is placed at too great a diflance to di- 



ftinguifh 



Let ABC reprefent the circular feftor, and DC a line bf- 

 fefling it ; the rod always returns to DC after the time of 

 ef 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 defcribing AE, 

 afid twice the time of defcribing EB; fo that two fuc- 

 (ceeding intervals of its returns are equal to the time of two 

 vibrations : but the intervals of the returns to the lines AC 

 or CB are manifellly equal to the time of two entire vibra- 

 tions. The brightnefs of the feflor therefore in DC, or any 

 line between DC and AB or BC, muft be fimply as the quan- 

 tity of light emitted from equal fmall portions of the feftor ; 

 that is, in the inverfe proportion of the velocities of the rod 

 when in thefe lines. It is plain from this, that the feftor 

 muft be incomparably 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 diftinftly with a white rod on each fide. 



If the rod be agitated with fmall and quick vibrations of 

 its own, by flicking it againft fpme folid body immediately 

 before it is hurried backwards and forwards with the angular 

 motion, the fedor appears divided, at equal intervals, by a 

 great many diftinfl rods, almolt as bright as the two lateral 

 ones (Tab. iii. Fig. 8.) refembling the fpokes of a fpread fan. 

 The reafon of which curious fkrrnomcnGn is plainly this ; that 

 its angular motion, being alternately in the fame and in a 

 contrary diredion to its particular vibrations, is alternately 

 accelerated and retarded or ftopt. In the interval, where it is 

 accelerated, the fedor muft appear very dilute ; and, whiere 



