466 Some Spectroscopic Notes. 
: Pe : ) 
The resolving power or — is thus given by 
dn 
w—1 *) 
ne PE me 
which is the value arrived at by the other method. 
Let us now consider the secondary maxima more closely. 
Their directions are given by n tan (H+F)= tan n(H+F), 
of. Preston. As E+ F increases from 0 to 7, we pass from the 
direction of one chief maximum to that of its neighbour. If 
we solve | 
y=ntan (E+F) 
y= tan n(h+F), 
ii+F varying from 0 to 7 in each case, the values of H+ F 
thus found give the directions of the secondary maxima 
within the range. The result for n=26 is approximately 
Dip Siri Aare 49a 
H+b= 5, 59? 59” e Meaieeite Boe 
Applying the formula for the relative intensity, we find that 
if the intensity of the chiet maximum be denoted by 1, that 
of the neighbourmg maxima should be 0°05, 0°02, &e., but 
all greater than 0°001. There are twenty-four of these 
secondary maxima between two adjacent principal maxima. 
5 T 
Two of these secondary maxima are as a rule — 
‘ 6 apart. 
ow 
ru 2s pales cas : : 
(H+ F) a5 (sin “i —w sin 2) +e(2 Cos 2— Cos pt) 
+ A@(f cos mite sin p). 
If the change in (E+F) is approximately a the corre- 
; yh ee 
sponding change in A@ is GF? and this is exactly the 

theoretical limit of the resolving power. These secondary 
maxima, even if bright enough, should not be seen as lines 
but should form a continuous background. When using 
the electric arc on some occasions, however, the faint back- 
ground between the orders appeared as if constructed of a 
great number of fine lines. 
The ratio of the distance between the principal and adjacent 
secondary maxima to the distance between two principal 
e . 3 . e ° . . 
maxima is 5-. The width of a principal maximum in a 
