applied to the Equatorial Injlr ament , See. 533 
fvvering to that altitude may be found by the common 
tables, and the concave lens fet accordingly, by means of 
the fcale at the fide, which is divided into half minutes, 
and, if required, by ufing a nonius, may be divided into 
feconds. 
It mull be obferved, that when a ftar or planet is but 
a few degrees above the horizon, the refraction of the 
atmofphere occafions it to be confiderably coloured. 
The refraction of the lens acting in a contrary direction 
would exaCtly correCt that colour, if the diflipation of 
the rays of light were the fame in glafs as in air; but as 
it is greater in glafs than in air, the colours occafioned by 
the refraction of the atmofphere will be rather more 
than corrected by thofe occafioned by the refraction of 
the lens. 
A drawing of the refraCtion apparatus is added, which 
may ferve to give a more clear idea of it. See plate IV. 
EXPLANATION OF THE PLATE. 
a a, The circular brafs tube, which fits on to the end of 
the telefcope. 
bb, The oblong concave lens in its frame, which Hides 
over the fixed convex lens. 
' Y y 2 
c, The 
