of Eye-gJaJfis for Telefcopes, £§► 
To give a proper demonftration and theorem for the exa£l 
form of the frit lens, according to its diftance from the image, 
would require more leifure than is confident with the fitua- 
tion of one not very converfant with mathematics. That 
diftance, in proportion to the focal length of the lens, fo that 
any unavoidable defeat in it may become invifible, muft he 
determined by experiment. If any variation be made in the 
form of this lens, it will be better to make the plane fide rather 
>a little convex than concave. By the latter the image would be 
diftorted by the too great obliquity of the rays near the extre- 
mity of the lens. 
Thus we have a fyftem of eye-glafifes which may be taken 
out of the telefcope, in order to wipe them at pleafure. Or 
the magnifying power of the telefcope may be varied without 
afie£ting the line of collimation, or in any manner altering the 
ad j uftmen t of the inftrument to which fuch telefcopes may be 
applied with many other advantages. In the prefent im- 
proved ftate of telefcopes too, the difiigreeable appearance of the 
wires from the great power of the eye-glaftes is in a great de- 
gree remedied. The fame principle may be ufefully employed 
in many other cafes. What is herein contained is only to be 
confidered as an explanation of this very ufeful conftru&ion, 
and which is given in hopes that fome perfon of more abilities in 
the fcience of optics will favour us with a general theorem, in 
order that its application may be more univerfal. 
