= 
On the Achromatic Microscope. 267 
a 
’ Inch. . 
Radius of the 1st surface, 0.16 si 
ped, os: .. 21. -de.. 0.23 epee 
do, Sd *. do. -..0.1GaDas | 
do. . 4th do. 0.24 Rint. 
do. 5th do. 0,24 
“do. ' 6th . do. 0.23 plate , 
do. 7th do. 0.12 Lk 
do, 8th do. 0.155 ‘ ip 
wos - th do.) Re fli 
Rte. Lh © do. O18 sais 
do. lth do. 0.18 2, 
do. 12th do. 0.155 ; pale 
a 13th do. 0.09 1 
do. 14th do. 0.08 plate 
do. 15th do. 0.08 int 
do. 16th do. — 0.12 5 i 
Sarde -¥7th. do, < 0.12 
do. 18th do. 0.09 plate 
Focus of the compound lens, 0.102 
Diameter of the Ist set 0.14%. 
do, Qd 0.12 | 
Rice. 88 8, ORE 
Clear apertire, 0.12 to 0.14 
_ Each of the three kinds shown in the figures has been constructed _ 
and found to perform well. The convex lenses of the object glass 
shown in Fig. 3, should be made as thin as is practicable, that the 
focus of the inicroscope may be as far as possible from the last sur- 
face. The thickness.of the flint lenses’ at their centres may be 
‘out +3 of an inch. If they are much thimer they will bend and 
become disfigured while polishing. : | 
The lenses of the object glass of an achromatic microscope must 
be centred with great accuracy; and they. may be considered as well 
*entred, when their optical centres do not vary from a right line more 
than Tote Of an inch. a 4 
Flint glass is preferable which has the greatest dispersive r 5 
for the secondary aberration of figure is less, when the aperture Is 
€ same in relation to the focal distance; although it is greater, 
When the aperture is the same in relation to the first radius of the 
flint lens. . 
