251 
determining the dispersive ratio of glass y ^c. 
p = 1-70 , q = 1-03 , p = 170 
p = 1-75 , q — 1-09 y p=il 3 
•05 : *06 : : -03 : *036 
Whence q = 1066. 
Then again r' =f a = 27-2 very nearly, 
r=fa{q-\- 1) =z 28-9. 
Hence the required radii are 
r = 28-9 convex ■> 
t ^ plate lens. 
r z=. 27-2 convex ) ^ 
r"— 27-19 concave 
r"'= 271-9 concave 
Comparison of the preceding formulce with the empirical rule 
said to be employed by Mr. Tully. 
15. According to the description we have, under the article 
Telescope in Rees’s Cyclopaedia, of the principle of computa¬ 
tion adopted by this ingenious optician, it appears that instead 
of computing the aberration of the flint lens from the focal 
point, it is calculated for parallel rays, and always for the 
index 1-500 ; the formula made use of being 
^_ 27 (p + 67 + 7 
r ~ 6 + I)-^ 
as first given by Huygens. This must necessarily give an 
erroneous result; and to correct it, a comparison of various 
experiments has led to the formation of an empirical multi¬ 
plier, which is said to compensate for the erroneous supposi¬ 
tion, and to have formed the ground work of the practice of 
this able artist. 
I 
flint lens. 
