ox A TABLE OF STANDARD WAVE LENGTHS OF THE srECTHAL TINTS. H 
from the normal spectrum due to the use of long plates. Tlic plates in the hi)\* an' 
bent to the arc of a, ch'cle of radius r. "When aflorwards straighleneJ \\c iiit'!i«nr<» 
the distance by a linear dividing engine. Hence, what we measure Is Die nic with 
radius r. Let a and /3 be* the angles of incidence and diffraelion fnmi the gratinjr. 
We have then to express /B in terms of S. Let X be the wave length, and ii and .V 
the number of lines on the grating to 1 mm. and the order of the ppretrnni n >pee(- 
tively. Then 
X = (sin a + sin ^) ; 
nN 
b ■ ( „ 5 
sin 5 = ^ sin -cos ( }' + ^ 
^ R '1 \ 
Li these formulae a is the angle to the centre of the photo-rnpbic plato, and fi 
and 8 are also measured from the centre, y is the angle between the radius Umx li.e 
centre of the photographic plate and the line drawn from that point to the centre of 
the grating. When properly adjusted, y will be zero. Also wc make 2r = //, 
to obtain perfect focus throughout. So that 
;.=:-L [sin« + Bin-). 
Callin- X„ the wave len-th at the centre of the plate, we have approximately 
^ /VQ l^iXV. »f«*V. XV.^^^ 
;_ ;.^ ^ _^_ !^\?. -?.)«+ etc. 
'2 11 N 6 
The first quantity 
8 
2nN 
f X-Xo 
be 
f the correc 
The last term is the required correction expressed in terms of the pr 
length. The correction in actual practice has been made from a plot 
tion on a large scale, and never amounted to more than a few hundredths 
division of Angstrom, even for the longest plate. 
In two or three plates the camera was displaced, so tliat y had 
cases no attempt was made to measure y, but the plates 
In snrh 
ily used for 
L v.^,nT« iispd as snbslandards. 
:.polation by drawing a curve through ccrt.ui iioui« ued 
These substandards were principally used for worlung up the la. .a l 
graphic pi 
the solar spectrum and 
pectra 
of 
higher orders, or both. Some of them contamed three metaUic spectr 
