Astronomy. — "On the magnitude equation of Osthoi''1'''s estimates 

 of star-coulouvs" . By E.inah Hkhtzsprung. 



(Communicated at the meeting of February 24, 1923). 



In Annalen van de Sterrewaclit te Leiden Vol. XIV, Part \, p. 

 14; 1922 I have noticed an iine.xplained magnitude e(|uation for 

 the derived c,/T values of stars of the spectral classes AO, A2, A3 

 and A5. Now the c.,/T values n.sed I.e. depend for about 58 percent 

 of the total weight on diiect colourestimates. A redetermination 

 of the magnitude e(piation of those estimates is theiefore very desir- 

 able. The opportunily for tins is given by the new catalogue of 

 OsTHOFi' (Specola Astrononiica Vaticana Vol. VIII; 1916) extending 

 liis estimates with the 4 inch refractor one magnitude farther viz. 

 to about 6'". A. card catalogue was made containing the hour of 

 R. A., the degrees of declination, the spectrum of the new Draper 

 Catalogue H.D. (taken from the Index Catalogue, Spec. Astr. Vat. 

 IX; 1917), the magnitude to one tenth and the estimated colour. 

 The cards were divided into groups according to spectrum. After 

 some trial tiie subdivisions of spectral class were combined in the 

 way as shown in Table 1. For each of the 6 combined groups 

 corresponding values of mean magnitude and mean estimated colour 

 are given. On the accompanying diagram the figures of Table 1 

 are represented graphically. 



The most striking fact is, that the estimated colour does not, as 

 hitherto adopted '), increase continuonsl)- with decreasing apparent 

 brightness but shows a maximum in the neighbourhood of 4'" or 

 5™. Especially for the white stars the decrease in estiituited colour 

 between 5'" and 6'" is very marked. This is nothing more, than 

 should be expected from the known peculiarities in colourconcep- 

 tion by the human eye. If the spectrum of the sun is made to in- 

 crease in intensity starting just below the limit of visibility, the 

 blue and green portion will appear first, but without showing any 

 colour, until by still greater intensity the colours green and blue are 



*) A. Panaekoek, Koninklijke Akademie van Wetenschappen te Amsterdam, 

 Proceedings of the Meeting of Saturday October 27, 1906, and E. Hertzsprung, 

 Zeitschr. fur wiss. Photographie Bd. 5, 100; 1907. 



