[ lr + 1 
but it is evident neither of thefe motions can be ac- 
curately performed. In the third method, the ob- 
ferver is only required to move both himfelf and the 
quadrant, l'o as to keep the Sun always in or near 
the axis of vifion, which may be performed very 
well, becaufe the axis of vilion is a vifible and cer- 
tain direction for it. One exception, however, Ihould 
be made to this general rule, namely, in taking the 
Sun’s altitude when very low, by the back obfer- 
vation ; in which cafe it will be bed: to ufe the fecond 
method, or elfe to hold the quadrant perpendicular 
by judgment; which will be much facilitated by 
tiling a teiefcope containing wires in its focus parallel 
to the plane of the quadrant, as defcribed in p. 106: 
for, in this cafe, the perpendicular pofition of the 
quadrant cannot be attained fo near by the method of 
turning the quadrant on a line going to the Sun as 
an axis, as it can by the other method. 
It remains to treat of the errors which may arife 
from a defeCt of parallelifm in the two furfaces of 
the index-glafs, and to point out the means of ob- 
viating them in the celeftial obfervations. It is well 
known, that if a pencil of parallel rays falls upon a 
glafs whofe two furfaces are inclined to one another, 
and fome of the rays are reflected at the fore-furface, 
and others palling into the glafs and fullering a re- 
flection at the back-furface and two refractions at 
the fore-furface emerge again from the glafs, thefe 
latter rays will not be parallel to thofe reflected at the 
fore-furface, as they would have been if the furfaces 
of the glafs had been parallel, but will be inclined to 
the fame. I find that the angle of their mutual in- 
clination, which may be called the deviation of the 
rays 
