determining the dispersive ratio of glass, &c. S 49 
IS. Table showing the aberration of a lens for parallel rays to 
the four different indices *1500, ‘1510, ’1520, ’isso. 
p zz the 
value of 
aberra- 
Ratio of surfaces, or values of ^ for different indices. 
1.500 
1-510 
5-20 
5’30 
tion. 
? 
? 
i 
1*05 
Imaginary. 
Imaginary. 
Imaginary. 
-180 
1*10 
•292 
-291 
•296 
•303 
115 
•380 
•374 
•377 
•380 
1*20 
•445 
•445 
.446 
•447 
!-25 
•5*5 
-510 
-506 
-509 
1-30 
•572 
•570 
•568 
•567 
*•35 
•635 
-627 
•625 
■626 
I -40 
•689 
-686 
•683 
•683 
I -45 
750 
•743 
•740 
740 
I -SO 
•803 
•800 
-798 
•798 
*■55 
•865 
-858 
•85s 
•8SS 
I '60 
•921 
-917 
•9*3 
•9*3 
1-65 
•979 
•975 
•973 
•972 
170 
I ’042 
1-037 
1-034 
1-034 
175 
1-103 
1-098 
1-096 
1-095 
I *80 
I-166 
i-i6i 
*•*59 
1-159 
1-85 
1-230 
1-226 
1-223 
1-223 
1-90 
1 -296 
1-292 
1-289 
1-290 
1*95 
1-360 
1-356 
*•357 ' 
*'359 
2’00 
*•434 
*•43* 
1-429 
s -430 
2-05 
1-506 
1-503 
1-503 
1-504 
2 'IO 
1-581 
1-578 
*•579 
1-5S0 
z-ts 
1-658 
1-657 
1-658 
1-659 
z-zo 
x-738 
1-738 
1-738 
1-741 
Example. 
14. Let it be proposed to compute the curves for an achro 
matic object-glass ; the data being as below, viz. 
Index of plate = 1-515, of flint = i'6oo, 
Dispersive ratio ™ -66, diameter 5| inches, focus 80 inches. 
Here a ™ '515, a*^ *600, d -66, f" 8©. 
First /" ( 1 — 80 K -34 “ 27’20 ss /, 
?i=yy-=4r2i=/'. 
Kk 
WJDCCCXXVII, 
