( z26 ) 
a Tinown and fundamental Principle in Opticks, that the 
Angle of Reff adion is equal to the Angle of Incidence ; 
wherefore tec the Rays BD B Dh^ imagined co come 
from fome point of an Objed: placed within a Spher of 
Water, by being Refleded from the interior Surface of 
the Spher at 5fi CBD is the Angfe of Refleaion, 
to which making C B E equal, fo will F be the place 
where an Objeca- fending forth a Cone of Rays, two of 
which are F B F B, which are Refledled into the Rays 
BP BDs and then coming to the other fide, iheSpbear 
at D and D, they are Refrafled into DE D E as be- 
fore, and confequently be as fit for diftin(9: Vifion, whe- 
ther the Objeft be placed in /^within or in ^without 
the Spher, if its interior Surface be confidered as a 
Concave Refleding Speculum. 
That the interior Surface of Glafe, and confequently 
of Water is Rcflefting, common Experience fliews ; 
but whether any Body have before taken Notice, that 
the Air is Specular is to me unknown : But I have, a 
very few Days fince,as I was endeavouring t^ improve 
this natural Catadioptrick Microfcope ftumbled upoiy an 
Aercal Concave Speculum which I fliall now defcribe. 
A Darknid Room being fomewhat troublefome to 
make, I thought it proper to try, if this Inconvenience 
might not be remedied fo 1 took a ftifF piece of 
brown Paper, pricking a finall hole therein, then ap- 
plied the Drop of Water to my Eye, and holding the 
Paper with the holeata little diflance before me, I could 
fee the Globules therein little lefs diflindHy then in a 
Darkned Rome : But before I had removed the Wa- 
ter, there appeared to me a very ftr^nge and furpri- 
fing Appearance ; I faw the Needles point together, 
with the Water inverted, I could fcarce at firft believe 
my 
