218 Mr. Bownoin on an all-furrounding Orb, 
concave mirror, whofe arc does not exceed fifteen or eighteen* 
degrees ; the’cylinder of rays falling upon it parallel to its axis, 
will, if there be no refraction, be: reflected to a focus round 
that axis ; the focus being nearly equidiftant. from. the pole of 
the fegment,: and the centre of its {phere : and that thofe rays, 
if previoufly refracted, and. clafled.into their feveral colours, 
will, in their divergence from the focal point, thew. thofe co- 
lours in a reverfed order : the refraction,. however, occafioning. 
an alteration in E dion. of the Mc oci A adi vie diverging. 
cone. ^ 
To apply ed of thefe obfervations,. it may’ * en -thar 
the interior fide of the expanfe has, in general, an uniform fur- 
face, which may be conceived as compofed of a multitude of 
fegments;. each of them not. exceeding a given arch : that it i$ 
. furnifhed with-an atmofphere, poflefling, in fome peculiar mode, 
the power of refracting light, of diftributing its rays into their 
refpective claffes, and tranfmitting them to the expanfe ; which 
alfo may be conceived as afffting, by its reflecting power, in 
their claffification ; that the tranfinitted rays would, in their 
claffed ftate, be reflected from it in all directions ; and. that 
fuch of them (by far the Smee part of the whole) as (hould 
come 
9% Gravefand?'s Natural Philofophy, Book III. ch. xv. prop. 8r3. 
+ That thefe parallel rays (parallel, I mean, to any and every conceivable di- 
ameter-line of the expanfe) muft conftitute the greateft quantity or proportion of 
theseflested light, ‘willbe manifeít: from thefe confiderations :—That they come 
Par gme t Ba of the expanfe from the oppofite part of it, and from the 
"between fuch oppofite parts : that the diftance of any two oppofite 
hee equal to the diameter of the expanfe, is the greateft that 
NE ai: that there mut, therefore, be, i in the fpace between them, 
most OF syvems füppl ne Fetes hh Het, than there cag be in 
any. 
