38 EXPLANATION oF an 
But this is not the cafe; for thofe parts which are fartheft 
from the eye in the object, will always be fartheft from 
the eye in the image, and often ina much greater propor- 
tion. After fome time I concluded it to be a neceflary 
confequence of the apparent inverfion of the objet; and 
many things tended to confirm me in this opinion, before 
{ made the experiments which feem perfectly decifive. 
It has often been matter of furprize to me, when view- 
ing the moon through a good telefcope, in company with 
perfons not accuftomed to fuch obfervations, that whilft_ 
the cavities and eminences of the moon’s furface appeared 
to me marked out with the utmoft certainty by their light 
and fhades, my companions generally conceived it to be 
a plain furface of various degrees of brightnefs. The rea- 
fon I fuppofe to be this; the aftronomer knows from the 
moon’s fituation with refpect to the fun, and even from 
the figure of its enlightened part, precifely in what direc- 
tion the light falls on its furface, and. therefore judges 
rightly of its hills and vallies, from their different degrees 
of light, according to thofe rules which are imperceptibly 
formed in the mind, and confirmed by long experience. 
But a perfon unacquainted with aftronomy knows nothing 
of the direction of the fun’s light on the moon, nor does 
he attend to the moon’s globular figure, and is befides, 
perhaps, pofleffed with a notion of its being felf-luminous ; 
no wonder then that the fame objec has a very different 
effe& on his imagination. It feems to be thofe rules of 
judging, which we begin to form in our earlieft infancy, 
which we fet afide, re-eftablith, alter, correé&t and confirm, 
and at length rely on with the utmoft confidence, even 
without knowing that we do fo, or that we have any fuch 
rules: It is thefe rules, of fuch infinite general ufe to us, 
that fometimes miflead us on new and extraordinary oc- 
cafions, and particularly in the cafe now before us. A 
perfon entering into a room perceives, at a fingle-glance, 
whence the light comes which illuminates the objects be- 
fore 
