“8 ESSAYS anp OBSERVATIONS 
\ 
an eye which is placed at too great a di- 
{tance to diftinguifh the feparate points. 
In whatever manner fenfation be perform- 
ed, it is certain, that the organs which 
receive the firft impulfe from external ob- 
jects cannot convey to us any ideas, if they, 
or the impreffions made by them, be lefs 
than of a certain definite magnitude. A 
number of things feparately intangible, 
if joined together, may be felt by the 
touch: A certain number of invifible 
points become fufficient to affect the fight 
by their united rays; and a certain num- 
ber 
the angular motion, the feétor appears divided, at equal 
intervals, by a great many diftin@ rods, almoft as bright 
as the two lateral ones (Tas. ili Fig 8 ) refembling the 
fpokes of a fpread fan, The reafon of which curious 
phenomenon is plainly this; that its angular motion, be- 
ing alternately in the fame and in a contrary direction 
to its particular vibrations, is alternately accelerated and 
retarded or (topt. In the interval, where it is accelerae 
ted, the fector muft appear very dilute; and, where it 
is greatly retarded or brought to reft, mult appear very 
luminous or divided by white rods, for the fame reafon 
that they appear at the fides, 
