_Colour_ _Sp._ _Number_ W, W+ A0 281 YW- A0 356 YW A5 482 YW+, YW- F3 211 WY G4 264 WY+, Y- K1 289 Y, Y+ K4 254 RY-, RY K5 85 -------------------------------- Sum ... 2222 The signs + and - indicate intermediate shades of colour. The preceding table drawn up by Dr. MALMQUIST from the colour observations of MÜLLER and KEMPF in Potsdam, shows the connection between the colours of the stars and their spectra. The Potsdam observations contain all stars north of the celestial equator having an apparent magnitude brighter than 7m.5. We find from these tables that there is a well-pronounced _regression_ in the correlation between the spectra and the colours of the stars. Taking together all white stars we find the corresponding mean spectral type to be A0, but to A0 corresponds, upon an average, the colour yellow-white. The yellow stars belong in the mean to the K-type, but the K-stars have upon an average a shade of white in the yellow colour. The coefficient of correlation (_r_) is not easy to compute in this case, because one of the attributes, the colour, is not strictly graduated (_i.e._ it is not expressed in numbers defining the colour).[5] Using the coefficient of contingency of PEARSON, it is, however, possible to find a fairly reliable value of the coefficient of correlation, and MALMQUIST has in this way found _r_ = +0.85, a rather high value. In order to facilitate the discussion of the relation between colour and spectrum it is convenient to deal here with the question of the spectra of the stars. 10. _Spectra of the stars._ In order to introduce the discussion I first give a list of the wave-lengths of the FRAUENHOFER lines in the spectrum, and the corresponding chemical elements. _FRAUENHOFER line_ _Element_ λ A 759.4 B 686.8 C(α) H (hydrogen) 656.3 D_1 Na (sodium) 589.6 D_3 He 587.6 E Fe (iron) 527.0 F(β) H 486.2 (γ) H 434.1 G Ca (calcium) 430.8 h(δ) H 410.2 H(ε) Ca(H) 396.9 K Ca 393.4 The first column gives the FRAUENHOFER denomination of each line. Moreover the hydrogen lines α, β, γ, δ, ε are denoted. The second column gives the name of the corresponding element, to which each line is to be attributed. The third column gives the wave-length expressed in millionths of a millimeter as unit (μμ).