ox A TABLE OF STANDARD WAVE LENGTHS OF TTTE SrECTIlAL LINES. 10*! 
Thus, using a line P to start with; 1 determine other groups of lin-s, a% //, c\ iT, 
etc. From these again I find groups, some of which may be (he same as tlic fiibt; 
then again from these, other groups. The process can be coniiuued further, but 
we are apt to come back to the same hues again, and we are further limited 
the visibility of the lines. Thus the limit of great accuracy by eye observation 
in either direction is practically 4200 and 7000; although in a dark room. <'s- 
pecially in the first spectrum, one can see much further, even bcy^oiid the -1 ^roiip, 
although it is difficult to set on the lines, and one is npt to misinke groups of 
lines for single lines.* When one uses a group as a standard, and one or m"!o 
of the group is an atmospheric line which varies, the measurers will of cnurse vary 
also, unless the atmospheric line is in the centre of the group. This is a v. ry 
common source of error, and has caused me much trouble. In a gratini: with a 
very bright second spectrum, I have, however, obtained the coincidence of A vvilli 
the reo-ion whose wave lencrth is about 5080, and have thus confirtncd (he vahie 
^.^w ... ..^ ,, -5 
given in my preliminary table, which was obtained by a very long In(crp<)la(ion 
passing from the first into the second spectrum. 
The accuracy of these primary standards Can be estimated from the equations 
given in Taljle VII. It is there seen that there is scarcely any difference in (he 
different measures as derived from different lines. 
It is to be specially noted that the wave length of F and the lines directly 
determined from it have no more weidit than any of the others. The table might 
just as well have been arranged with the D line, or any other, first. The true 
way of discussing the results is to form a series of linear equations, about 
all. and solve them. This is the method I have used, although I have not 
twontv- 
six in 
Id 
discussed them by the method of least squares.! 
Some miscellaneous observations not included in the table allowed me to n 
■ 
a few more lines to these primary standards. 
Having completed these primary st^xndards, I then observed severa hundred 
standard lines in the visible spectrum, including these primary standards ^^it.ti a 
micrometer having a ran^e of five inches, and very accurately made. he spcc- 
^nnnrtional tO tllC WaVC 
o o 
trum being strictly normal, the readings so made were p 
length. They could have been used simply to ^^^^erpolate^ between jlie^^n 
standards, but I preferred another method: The readnigs 
of the micrometer were 
♦ In a 
1 ^oncnrpd liiT^^ flown to wavo If 
very bright grating I have faintly seen, and even moasu cd, Un ^ 
. / r. T. „J, „„ ° „„ ... u.^. wh.t. i.s called the ultra violet, even to «a... 1 n tn 
'.00. .My 
bev 
assistant, Mr. L. E. Jewell, can see far into what is called the ultra vio , - ^^_^^^ personally uritt'^n 
t The calculations of this paper have involved about a auihon figure,, of 
more than half. Hence I am not anxious for more labor of this Icma. 
v 
