64 ESSAYS anp OBSERVATIONS 
-cdlours; and it is no lefs fo, that, fince he 
quitted the fubject, no further difcovery 
of any moment has yet been made a- 
mongft all the philofophical focieties in 
Kurope. 
3 52. 
with compleat luftre ; Since they fend entire pencils of 
rays EKF, ELF to the eye: And the vifible brightnefs of 
every point trom L towards M mutt decreafe gradually , 
as from K to N: d. e. The fpaces KN, LM will appear as 
dim fhadowy borders or fringes adjacent to the edges of 
the opaque bodies. When the edge G is brought to’ 
touch the right line KF, the penumbra’s unite; and as 
foon asit reaches NDF, the above phenomenon begins: 
For it cannot pafs that right line without meeting fome 
line aDd drawa from a point between N and K, and, 
by intercepting all its rays that fell upon the pupil, ren- 
der it invifible. In advancing gradually tothe line KDE, .- 
it will meet other Jines Df, cDg, &c. and therefore ren- 
der the points b, c, &c, from Nto K fucceflively invifi- 
Ble; and therefore the edge of the fixed opaque body 
€D muft feem to fwell outwards, and cover the whole 
fpace NK, while GH by its motion covers MK. When GH 
is put to a greater diftance from the eye, CD continuing 
fixed the ipace OP to be paffed over for intercepting 
NK islefs; and therefore, with an equal motion of GH, 
the apparent {welling of CD muft be quicker ; which is 
found true by experience, If ML reprefents a luminous 
object, and REFQ any plane expofed to its light; the 
fpace FQ will be entirely fhaded from the rays, and the 
fpace 
