ASTRONOMY: IV. S. ADAMS 
145 
which show but a moderate change of intensity in these types, and es- 
timates are made on an arbitrary scale, extending from zero to ten, of 
the differences in intensity between the hydrogen lines and this selected 
list. The calcium Hne X 4227 is also compared with By in the types 
FO to G5, beyond G5 the differences becoming too great to provide sat- 
isfactory determinations. The list of pairs of Hnes finally adopted for 
classification purposes is given in Table I. 
TABLE 1 
Lines Compared Range of Type 
//7 and X 4227, Ca FO to G5 
Hy " X 4326, Fe F3 to Ma 
Hy " X 4352, Fe, Mg FO to Ma 
Hy " X4383, FO to G5 
Hy " X 4405, Fe F3 to Ma 
HI3 " X 4872, Fe GO to Ma 
H0 " \ 4:957, Fe GO to Ma 
The scale of classification was adapted to the Harvard system by 
selecting a considerable number of stars for which Harvard determina- 
tions were available, and making estimates of the relative intensities of 
these pairs of lines in the stars selected. The values were then plotted 
against the average types of these stars, and smooth curves were drawn 
through the several points. These curves provide the means of con- 
verting determinations of relative line intensity into determinations of 
spectral type. The curves are shown in figure 1. For reasons which 
will appear later, they are based upon stars of large proper motion alone, 
and the material may, therefore, be regarded as homogeneous in char- 
acter. 
To illustrate the use of these curves I have selected as examples the 
stars Groom. 3357, Piazzi 0^30, Groom. 145 and Lai. 19022. The es- 
timated differences of intensity for these stars, as determined from three 
photographs of their spectra, are given in Table II. 
TABLE n 
4226 4326 Ey Hy Hy El3 
Hy Ey 4352 4383 4405 4872 4957 
Groom. 3357 0.0 -5.3 +7.0 +4.3 +7.0 
Pi0^130 +3.7 +0.7 -1.3 +3.3 +1.0 
Groom. 145 +5.3 -2.0 -4.0 -0.2 -1.8 
Lal.19022 +6.3 -3.3 -5.0 -1.3 -3.3 I 
With the aid of tables constructed from the curves we obtain the fol- 
lowing determinations of spectral type from the separate pairs of lines: 
Mean Probable Error 
Groom. 3357 F7 F7 F4 F6 F5 F6 ±1.0 
Pi0^130 G7G5G6G4G5 G5 0.8 
Groom. 145 Kl Kl K3 K3 K2 K2 0.6 
Lal.19022 K7 K7 Ma K6 K6 K7 1.6 
