Computation of Tolies' i Objective. By Prof. B. Keith. 143 
The use of water instead of balsam, and perhaps the “ setting ” of the lens 
next behind the front, will slightly reduce the angle above given, but not by any 
material amount. 
The objective, as will be seen in the figure, is composed of seven lenses, 
Tjettering them in order, from the back towards the front, the elements, as given 
by Mr. Tolies, are as follows : — 
a 
d 
e 
/ 
3 
Refractive index 
1’525 
1’66 
1 
’5’25 
1*525 
1*62 
1*525 
1*55 
Radius of 1st surface . . 
0’52 
0’40 
GO 
0*35 
! 0*35 
0 23 
0*029 
2nd „ .. .. 
Thickness at centre . . 
0’40 
GO 
1 
80 
0*35 
00 
00 
GO 
0’132 
0’02 
0 
’045 
0*145 
0*03 
0*155 
0 025 
Diameter 
0’46 
0’46 
0 
’46 
0*43 
0*45 
0*22 
0*053 
The distances between c and d and between / and q are given by Mr. Tolies 
as very small, but not recorded in his memorandum; I have taken for the first 
O’ 01 and for tlie second O’ 006 of an inch. The adjustable distance between 
e and / Mr. Tolies gives as 0’07 when adjusted for a cover 0’ 01 4 of an inch thick. 
I have used 0’065 of an inch, which slightly increases the aperture, and is theo- 
retically coiTect for balsam contact, or a cover 0’016 of an inch thick. 
