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I. A Catoptric Microfcope. By Robert 
Barker, M. D. F. R S. 
T HOUGH Microfcopes, compos’d of Re- 
fracting Glades only, have been vaftly im- 
proved, as to their Effects of magnifying; yet 
they have been attended with fuch great Inconveni- 
ences, that their Application to many Arts, in 
which they might be very convenient, is not fo 
common as might be expected, and Mankind have 
reap’d but a fmall Part of the Advantage obtaina- 
ble from fo furprizing and ufeful an Inftrument. 
Among the Inconveniences mentioned, thefe 
are the mod: confiderable : 
, * . * • * X 
i. That in order to magnify greatly, it’s necef- 
fary the Object-Glafs be a Portion of a very mi- 
nute Sphere, whofe Focus being very fhorr, the 
Object muff: be brought exceeding near ; it will 
therefore be (haded by th z Microfcope, and not 
vifible by any other Light than what pafTes 
through itfelf ; in this Gale therefore, Opake Ob- 
jects will not be feen at all. 
2. Objects illuminated this way, may be rather 
faid to eclipfe the Light, than to be truly feen, 
little more being exactly reprefented to the Eye, 
than the Out-line j the Depreffions and Elevati- 
ons within the Outline appearing like fo many 
Lights and Shades, according to their different De- 
gree of Thicknefs or Tranfparency ; though the 
L I contrary 
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