the T y'lgonometncal Operation . i j ^ 
under the head of each. And in the uppermoft figure there 
appears a flat brafs ring, foldered to the infide of the tube 
about half an inch from the outward end, which carries on its 
furface four ftuds to receive the lower extremities of the four 
fcrews. Thus the grooves allow room for a fmall circular mo- 
tion to be given to the end-piece for the vertical adjuftment of 
the fork of the wires, thofe that are horizontal being by con-' 
ftrudtion at right angles with it. This being done, the fcrews 
are made very fall in the ftuds below, and thereby the whole 
machinery of the end-piece is rendered perfedtly firm and 
fee u re. 
There remains yet one piece more to be barely mentioned. It 
is the prifm eye-tube, reprefented by dotted lines towards the 
right-hand fide of Plate VI. as attached to the eye dnd of the 
tranfit telefccpe, inftead of the common eye-piece with two 
convex glafles. In leaning over our inftrument to obferve the 
pole ftar, highly elevated in thefe latitudes, the body is necef- 
farily thrown into an inconvenient fatiguing pofture, whereby 
fome rilk is run of deranging the inftrument, and confequently 
of making the obfervations lefs accurately than when the ob- 
ferver can look diredtly forward, without bending the body fo 
much. For this purpole, Mr. Ramsden promifed to fupply 
the prifm tube in 1787 ; but it was only, and with great dif- 
ficulty, obtained in 1788, by which time Mr. Dalby had 
accuftomed himfelf to obferve very well without it, fo that it 
was never ufed. 
By employing this piece, light is no doubt loft ; becaufe the 
image paftes through more glafles before it reaches the eye, than 
when the common eye-piece is ufed. But for obfervations of ftars 
nearer the zenith than the pole ftar is in our latitudes, it would 
be indifpenfably neceflary. It would likewife be advantageoufly 
X a ufed 
