NEWTON. 
from a tree, led his thoughts upon the sub- 
ject of gravity; and, reflecting on the 
power of that principle, he began to consi- 
der, that, as this power is not found to be 
sensibly diminished at the remotest distance 
from the centre of the earth, to which we 
can rise, neither at the tops of the loftiest 
buildings, nor on the summits of the highest 
mountains, it appeared to him reasonable 
to conclude, that this power must extend 
much further, than is usually thought. — 
“ Why not as high as the moon ?” said he 
to himself; “ and if so, her motion must be 
influenced by it ; perhaps she is retained in 
her orbit by it ; however, though the power 
of gravity is not sensibly weakened in the 
little change of distance at which we can 
place ourselves from the centre of the earth, 
yet it is very possible that, at the height of 
the moon, this power may differ in strengtli 
much from what it is here.” To make an 
estimate of what might be the degree of this 
diminution, he considered with himself, that 
if the moon be retained in her orbit by the 
force of gravity, no doubt the primary 
planets are carried about the sun by the 
like power ; and by comparing the periods 
of the several planets with their distances 
from the sun, he found, that if any power 
like gravity held them in their courses, its 
strength must decrease in the duplicate 
proportion of the increase of distance. This 
he concluded, by supposing tliem to move 
in perfect circles, concentric to the sun, 
from which the orbits of the greatest part 
of them do not much differ. Supposing, 
therefore, the force of gravity, when extend- 
ed to the moon, to decrease in the same 
manner, he computed whether that force 
would be suflacient to keep the moon in her 
orbit. 
In this computation being absent from 
books, he took the common estimate in use 
among the geographers and our seamen, 
before Norwood had measured the earth, 
namely, that sixty miles make one degree of 
latitude; but as that is a very erroneous 
supposition, each degree containing about 
sixty-nine and one-third of our English miles, 
his computation upon it did not make the 
power of gravity, decreasing in a duplicate 
proportion to the distance, answerable fo 
the power which retained the moon in her 
orbit; whence he concluded, that some 
other cause mpst at least join with the ac- 
tion of the power of gravity on the moon. 
For this reason he laid aside for that time, 
any further thoughts upon the matter. Mr. 
. Whiston (in his Memoirs, p, 33.) says, he 
told him that he thought Des Cartes’s vor- 
tices might concur with the action of gra- 
vity. 
Nor did he resume tliis enquiry on his 
return to Cambridge, which was shortly 
after. The truth is, his thoughts were now 
engaged upon his newly projected reflecting 
tetecope, of which he made a small speci- 
men with a metallic reflector spherically 
concave. It was but a rude essay, chiefly 
defective by the want of a good polish for 
the metal. This instrument is now in the 
possession of the Royal Society. In 1667, 
he was chosen fellow of his college, and 
took the degree of master of arts. And in 
1669, Dr. Barrow resigned to him the ma- 
thematical chair at Cambridge, the business 
of which appointment interrupted, for a 
while, his attention to the telescope ; how- 
ever, as his thoughts had been for some 
time chiefly employed upon optics, he made 
his discoveries in that science the subject 
of his lectures for the first three years after 
he was appointed mathematical professor: 
and having now brought his theory of light 
and colours, to a considerable degree of 
perfection", and having been elected a Fel- 
low of the Royal Society, in January 1672, 
he communicated it to that body, to have 
their judgment upon it; and it was after- 
wards published in their Transactions, viz. 
of February 19, 1672. This publication 
occasioned a dispute upon the truth of it, 
which gave him so much uneasiness, that 
he resolved not to publish any thing further 
fora while upon the subject; and in that 
resolution he laid by his optical lectures, 
although he had prepared them for the press. 
And the analysis by infinite series, which 
he had intended to subjoin to them, unhap- 
pily for the world, underwent the same 
fate, and for the same reason. 
In this temper he resumed his telescope : 
and observing that there was no absolute 
necessity for the parabolic figure of the 
glasses, since, if metals could be ground 
truly spherical, they would be able to bear 
as great apertures as men could give a polish 
to, he completed another instrument of the 
same kind. This answering the purpose so 
well, as, though only half a foot in length, 
to show the planet Jupiter distinctly round, 
with his four satellites, and also Venus 
horned, he sent it to the Royal Society, at 
their request, together with a description of 
it, with further particulars; which were 
published in the Philosophical Transactions 
for March, 1672. Several attempts were 
also made by that society to bring it to per- 
