end its Utility in the Aris. 19% 
ternhining the longitude at fea. In the latter labour I have 
been well feconded by Carrochez and other artifts who 
worked under my direction. 
_ Being fully fatisfied with the trials, of which I have given 
an account in the preceding memoir, I was defirous of con- 
- ftruéting a telefcope like that of Herfchel. For this purpofe, 
T engaged in the year 1790 a friend of the arts and fciences, 
the unfortunate Trudaine junior, to procure from Herfchel 
a good feven-feet telefcope. The fuperiority of the telefcopes 
made by that able aftronomer depends much more than ge- 
nerally believed on the Newtonian conftrution which he has 
adopted. The Gregorian telefcopes are by no means to be 
compared to the former. An appearance of convenience 
made them however to be commonly adopted, though that 
cenftruétion prefents great difficulties to. be furmounted in 
the complete execution of it. But as it is always neceflary 
in the ufe of telefcopes, the magnifying powers of which are 
confiderable, to adapt a magnifier that embraces a wide field, 
in order to find readily in the heavens the ftar to be ob- 
feryed, it may eafily be perceived that it is a matter of in- 
_ difference whether the telefcope employed for that purpofe 
be of the Gregorian or Newtonian conftruction ; becaufe in 
that cafe a direét or lateral view of the object is, abfolutely 
indifferent. The lateral fight is indeed much more conye- 
nient when the Newtonian telefcope is well difpofed on its 
‘ftand. Herfchel has adopted for his telefcopes a conftruc- _ 
tion ftill more ancient and fimples and though it approaches 
near to the origin of the art, it is no lefs ital when ufed 
for large inftruments. I have admired the effects of it in 
his telefcope of twenty feet; and,I fhould have been enabled 
‘ to form a better ate of it in that of forty feet, but 
there then {till remained fome improvement to be made in 
"the latter, which is unique in the annals of aftronomy. It 
is certain that a fmall inclination given to a.concaye mirror 
ofa long focus does not fenfibly disfigure the image formed 
-gmits focus. This fmall inclination is fufficient to permit 
the obferver to look through a flrong magnifier at the image 
of 
