[ 3°4 ] 
fering in colour. In water infedfs, as lobfters and 
fhrimps, thefe particles are white ; in fome land in- 
fers, as the caterpillar and the grafs-hopper they ap- 
pear of a faint green, when in the veiTels as I am 
perfuaded from experiments. I have feen them in an 
infedt no bigger than a pin’s head, and fufpedt they 
exift altnofc univerfally through the animal kingdom. 
What is fo generally extended through the 
creation mu ft be of great importance in animal 
(Economy, and highly deferving the attention of 
every enquirer into the works of nature. This 
fubjedt becomes the more interefting from fo much 
of reafoning in the theory of medicine being built 
in the properties of thofe particles. 
It is by the microlcope alone that we can dif- 
cover thefe particles ; and as fome dexterity and 
pradtice is required in the ule of that inftrument, 
there have not been wanting men of chara&er and 
ingenuity, who, having been unfuccefsful in their 
own experiments, have queftioned the validity of 
thofe made more fortunately by others. Some have 
gone fo far as to afferr, that no credit can be given 
to microfcopes, that they deceive us by reprefenting 
objects different from what they really are. Thefe 
afl'ertions, though not entirely without foundation, 
when we fpeak of one fort of microfcopes, are very 
unjuftly applied to them all. In compound mi- 
crofcopes, when the object is viewed through two 
or more glaffes, if thefe glafies be not well adapted 
to the focus of each other, the figure of the objedt 
may be diftorted; but no fuch circumftance takes 
place, when we view an objedt through a fingle 
lens. All who ufe fpedtacles agree, that the figures 
of objedts appear the fame through them, as they 
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