161 
The President's Address. By E. ill. Nelson. 
The lens is therefore constructed. It will he seen that s' has the 
same numerical value as r, and s half the value of s, in the preceding 
example. 
Case 4 . — Let it be required to construct a triple consisting of two 
biconvex lenses enclosing an equiconcave (fig. 17).* 
Fig. 16 . Fig. 17. Fig. 18. 
This is composed of two precisely similar doublets of the form in 
Case 2 (fig. 15), placed with their plane surfaces in contact. But as two 
doublets each of F = 1*0 would make a combination whose F = 1/2 it 
is necessary in order to keep the focus of the triple 1 • 0 to multiply all 
the radii by 2. 
Case 5. — To construct a Steinheil triple (fig. 18). 
Here we have two doublets with their plane surfaces in contact, of 
the form in Case 3 (fig. 16). 
The procedure in order to make the focus of the triple 1 • 0 is pre- 
cisely similar to the preceding case, viz. to double all the radii. 
Therefore w ith the above mentioned glasses a triple of the form in 
fig. 17 of 1 * 0 in. nominal focus can be constructed by making all the 
radii *8 in., and one of the form in fig. 18 by making the radii of 
the exterior surfaces * 8 in., and those of the contact surfaces half as 
much, or * 4 in. 
Example 6. — It is required to construct a triple of the form in 
fig. 17 of Chance’s Hard Crown and Dense Flint (Jena catalogue 
Nos. 8 and 36 respectively). Here we have for No. 8, /x = 1*5179 
and v = 60 * 2, and for No. 36, // = 1*6202, v = 36*2. n = 5^? 
iJU * iL 
= *6013; n = reciprocal = 
6013 
1*663 ; / = 1 - 
= *3987 and f = 1 - 1*663 = - *663. a = *3987 x *5179 
= *2065; b = - *663 x *6202 = - “4112. 
Let r" s", r s, r' s' be the radii ; then as in Case 2 we have s" = b 
= - -4112 ; and r" = - - S " 
a -j- s 
— *8401 
as" = - -8491; a + *" = - -2047; r" = — = -415. 
* A solid eye-piece for a telescope formed by a lens of this construction was 
exhibited in London at the Great Exhibition, 1851, by Mr. Reade. 
