1615 
The President's Address . By E. M. Nelson. 135 
ficienfc size to exhibit the error, they were too large for synoptical 
purposes, and smaller than that they were simply useless. 
Fig. 2G. 
Gentlemen, I thank you for the very kind attention you have 
given me while discussing this very dry and far from popular subject. 
Believe me, however, it is one that is of primal importance in optics, 
and, ex necessitate, one closely connected with microscopy. ‘ 
Formula. 
Let L, = 1000 
N = 
f 
3 ‘ 5 Ol> 2 “ c) 
z = 5^0*-/O 
Lpjj — L 0 2 
Y = 
{38-2(L a5 ^L D2 )+ *12} 
(N - 17 L x ) 2 
For lines higher up the spectrum than D 2 (towards the blue) 
/** = /h> a + Y + Z. 
For lines lower down the spectrum than C (towards the red) 
^ “ Z - 
In the last Table the asterisk stands for four cyphers, and the dagger 
for three. Example : Hard Crown F line reads, — • 000028 ; and Light 
Flint H x line, + ’000123. 
655/ 
