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LIGHT. 
est dust they meet with ; and the sun ap- 
pears to continue of his ancient dimensions, 
and his attendants move in tlieir ancient or- 
bits.” He therefore conjectures, that all 
the phenomena of light may be more pro- 
perly solved, by supposing all space filled 
with a subtle elastic fluid, which is not visi- 
ble when at rest, but which, by its vibra- 
tions, affects the fine sense in the eye, as 
those of the air affect the grosser organs of 
the ear; and even that different degrees of 
tlie vibration of this medium may cause the 
appearances of different colours. Franklin s 
Exper. and Observ. 1769, p. 261. 
The celebrated Euler has also maintain- 
ed the same hypothesis, in his “ Theoria Lu- 
cis et Eolorum.” In the summary of his 
arguments against the common opinion, re- 
cited in Acad. Bcrl. 1752, p. 271, besides 
the objections above-mentioned, he doubts 
the possibility, that particles of matter, 
moving with tlie amazing velocity of light, 
should penetrate transparent substances 
with so much ease. In whatever manner 
they are transmitted, those bodies must 
have pores, disposed in right lines, and in 
all possible directions, to serve as canals 
for the passage of the rays ; but such a 
sti'ucture must take away all solid matter 
from those bodies, and all coherence among 
their parts, if they do contain any solid mat- 
ter. 
Among modern philosophers who have 
supported this doctrine. Dr. Young has 
shown much ability in his experimental and 
theoretical researches, in his memoirs in the 
Philosophical Transactions,” which have 
been republished in his “ Lectures,” and in 
‘‘ Nicholson’s Journal.” 
The expansion or extension of any 
portion of light is inconceivable. Dr. 
Hook shows, that it is as unlimited as 
the universe, which he proves from the im- 
mense distance of many of the fixed stars, 
which only become visible to the eye by 
the best telescopes. “ Nor,” adds he, “ are 
they only the great bodies of the sun or 
stars that are thus liable to disperse their 
light through the vast expanse of the uni- 
veise, but the smallest spark of a lucid body 
must do the same, even the smallest glo- 
bule struck from a steel by a flint.” 
The intensity of different lights, or of the 
same light in different circumstances, affords 
a curious subject of speculation. M. liou- 
guer, Traite de Optirpie, found that when 
one light is from sixty to eighty times less 
than another, its presence or absence will 
not be perceived by an ordinary eye ; that 
the moon's H^t, when she is 19° 16' high 
above the horizon, is about one-third of her 
light, at 66“ 11' high; and when one limb 
just touched the horizon, her light was but the 
2,000th part of her light at 66“ 11 high; and 
that hence light is diminished in the propor- 
tion of three to one, by traversing 7,469 
toises of dense air. He found also, that the 
centre of the sun’s disc is considerably more 
luniinous than the edges of it; whereas 
both tlie primary and secondary planets 
are more luminous at their edges than near 
their centres ; that, further, the light of the 
sun is about 300,000 times greater than 
that of the moon ; and therefore it is no 
wonder that philosophers have had so little 
success in their attempts to collect the light 
of the moon with burning glasses; for, 
should one of the largest of them even in- 
crease the light 1,000 times, it will still 
leave the light of the moon in the focus ot 
the glass, 300 times less than the intensity 
of the common light of the sun. 
Dr. Smith, in his optics, vol. i. p. 29, 
thought he had proved that the light of the 
full moon would be only the 90,900th part 
of the full day- light, if no rays were lost at 
the moon. But Mr. Robins, in his Tracts, 
vol. ii. p. 225, shows that this is too great 
by one half. And IMr. Michell, by a more 
easy and accurate mode of computation, 
found that the density of the sun’s light on 
the surface of the moon, is but tire 45,000th 
part of the density at the sun; and, that 
therefore, as the moon is nearly of the same 
apparent magnitude as the sun, if she re- 
flected to us all the light received on her 
surface, it would be only the 45,000th part 
of our day-li;^ht, or that which we receive 
from the sun. Admitting, therefore, with 
M. Bouguer, that the moon's light is only 
the 300,000th part of the day, or sun’s light ; 
Mr. Michell concludes that the moon re- 
flects no more than between the 6th and 7th 
part of what she receives. 
Sir I. Newton long ago observed, tliat 
bodies and light act mutually on one an- 
other ; bodies on light, in emitting, reflect- 
ing, refracting, and inflecting it ; and light 
on bodies, by heating them, and putting 
their parts into a vibrating motion, in which 
heat , principally consists. For all _fixed 
bodies, he observes, when heated be- 
yond a certain degree, do emit light and 
shine. 
This action of bodies on light is found to 
exert itself at a sensible distance, though it 
always increases as the distance is diminish- 
ed, as appears very sensibly in the passage 
