C H A P . II.
Of V ision, of the O ptical E ffect of M icroscopes^,
AND OF THE M A N N E R OE E STIMATIN G THEIR M A G r -
nifying Powers.
THE progrefs that Has been made in the feienee o f optics,.
in the laft and- prefent century, particularly by Sir Ifaac:
Newton, may with propriety be ranked among the greateft acqui-
fitions of human knowledge. And Meff. Délavai and Heifchel.
have fhewn by their difeoyeries, that the boundaries o f this;
feienee may be confiderably enlarged.
The rays of light, which minifter to the fenfe o f fight, are the*
moll wonderful and aftonifhing part of the inanimate creation ; o f'
which we fhall foon be convinced*, if we confider their extreme
minutenefs, their inconceivable velocity, the regular variety of.
colours they exhibit, the invariable laws according to which they,
are afted upon tty other fubftances, in. their refleftions, infledtions,
and refraftions, without the leaft change o f their original properties
; and the facility with which they pervade bodies o f the
greateft denfity and clofeft texture, without reliftance, without
crouding or difturbing each other. Thefe,I believe, will be
deemed fufficient proofs o f the wonderful nature o f thefe rays ;
without adding, that it is by a peculiar modification o f them, that
we are indebted for the advantages we obtain by the microfcope.
The feienee of optics, which explains and treats o f many o f
the properties o f thefe rays o f light, is deduced from experiments,
©n which, all philofophers are agreed. It is impoflible to give an
adequate idea of the nature o f vifion, without a knowledge of
thefe experiments, and the mathematical reafoning grounded
upon, them; but as to do this would alone fill a large volume, I
fhall only endeavour to render fome o f the more general principles
clear, that the reader, who is unacquainted with the feienee
®f optics, may neverthelefs be enabled to comprehend the nature
©f vifion by the microfcope. Some o f the mod important of
thefe principles may be deduced from the following veiy
interefting experiment.
Darken a room, and let the light Be admitted therein only by
a fmall hole; then, i f the weather is fine, you will fee on the
wall, which is facing the hole, a pifture o f all thofe exterior ob-
jefts which are oppofite thereto, with all their colours, though
thefe will be but faintly Teen, The image of the objefts that are
ftationary, as trees, houfes, &c. will appear fixed; while the
images o f thofe that are in motion, will be feen to move. The
image o f every objeft will appear inverted, becaufe the rays crofs
each otherin puffing through the fmall hole. I f the fun fhmes on
the hole, we fhall fee a luminous ray proceed m a ftrait line, and
terminate on the wall. If the eye is placed in this ray, it will be m
a right line with the hole and the fun: it is the fame with every
other objeft which is painted on the wall. The images o f the objefts
exhibited on the fame plane, are fmaller in proportion as
' ©bjefts are further from the hole.
Many