156 Gen. Roy’s Account of 
ufed in looking at the meridian fun in fummer, for which pur- 
pofe it is furnilhed with dark giaffes, placed in a Aide moved 
by rack-work, as may be feen from infpeftion of the plate. 
They confift of three prifms, laid clofe to each other, fo as to 
form, when thus affembled, a parallelopipid. Here the green 
prifm ftands neareft to the eye, a dark one farthest from it, and 
between the two, one of white flint glafs, for correftion of 
the refraction which would otherwife take place. It will eafily 
be conceived, from the difpofition of the prifms, that the 
darkeft medium is here towards the left; and that it becomes 
gradually lighter towards the right hand, where a void part in the 
frame is brought into the field when the ftars are obferved ; or 
when, from the circumftances of the weather, it maybe unne- 
ceflary to fcreen the eye from the fun’s rays. 
Art. XVIII. General management of the inftrument for 
obfervation. 
W hen the inftrument is ufed on the ground, it is covered 
from the weather, under a circular tent, eight feet in diame- 
ter. Four Ihort piles, hooped and fhod with iron, are driven 
into the earth, and their heads levelled, by laying acrofs from 
one to the other a mahogany ftraight ruler, having a fpirit 
level attached to one fide of it. The feet of the Hand being 
then placed on piles, are firmly fattened to them by means of 
long fquare-headed fcrews, only one of which may be feen in 
the view of the inftrument, belonging to that foot which ftands 
neareft the eye. By working with the four fcrews fixed in the 
oft agonal mahogany plane, the plummet fufpended from the 
center of the inftrument is brought accurately over the point 
on the ground that marks the ftation. The fcrews of the feet, 
with the fide nuts appertaining to them, are then flackened, 
to 
