I 3 7

M.Z.U.11 J. D. 1 I 1

9h45" 2417109d41 '

11.34 8 ' 5 8 0 12 .33

18.35 8 25 8 0 21.33 7 35 , 35.32

4089

Schstzung

N I @

N I @

N I a a 3 N2.5 b a 2.5 N4.5 b a 6 N 1 b

Beobachtungen der Nova (104.1905) Aquilae am Utrechter Zehnzol ler .

Als die Beobachtung der Nova (104.1905) Aquilae hier angefangen werden konnte (Sept. 2 0 ) , war der Stern schon auf die elfte Grofie herabgesunken. Die Koordinaten und Helligkeiten der Vergleichsterne sind :

* I a 1855.0 I d 1855.0 I H. I H.C.

11.1

Die Kolumne H. C. gibt die Bezeichnung des Sterns im Harvard Circular No. 106 (A. N. 169.391); der Stern G kommt darin nicht vor. Ich ziehe die Harvarder Hellig keiten den von J. A. Parkhurst (Astroph. Journ. XXII S. 269) gegebenen vor. Der Stern a ist namlich im 32811. Sucher unsichtbar und dtirfte deshalb wohl SII?O sein, wahrend der Stern b nach meinen Schatzungen etwa om7 oder om8 schwacher ist als a.

Hier folgen die im 10 ZGller angestellten Stufenschat- zungen (vierte Spalte) nebst den daraus hergeleiteten Hellig- keiten (funfte Spalte) der Nova,

'905

Sept. 2 0

2 3 2 9

Okt. 8 B 16

25 Nov. 16

> 24

> 2 2

Bemerkungen

Nebef ; unsicher

Der sehr grofie Stufenunterschied a N = 8 am 25. Oktober wurde mittels eines hier nicht weiter bertlcksichtigten Zwischensterns geschgtzt.

Utrecht, 1905 Dezember. A. A. NqZand.

Determination of radial motions by objective prisnis. (Harvard Col lege Obse rva to ry Circular No. 110).

In Harvard Circular 13 (A. N. 3391) a method is described for de- termining the radial motion of stars from spectra obtained with objective prisms. As this method has recently been criticized, it may be well to describe some recent results obtained here, with the Draper telescopes. A photograph is taken in the usual way, the prism is then turned 180°, and a second exposure is given on the same plate. It is not necessary or advisable to use two plates, as re- commended in Circular 13 (A. N. 3391). It is often more convenient to reverse the telescope, instead of turning the prism. In the latter case, the plate must also be turned 1 8 0 ~ . The second images should be brought side by side with the first, and as near together as possible. It may be better to place the spectra end to end. They may be brought into any desired position by the aid of a Fig. I.

ground glass screen, or more pre- cisely by an eyepiece with cross-hairs. Were there no errors, it would only be necessary to measure the distance apart of the corresponding lines of each pair of spectra. Each star whose radial motion was known, would serve to determine the constant distance. apart of the lines. The differences in distance, converted into wave lengths, would give the required motions of the other stars. The motions of the Sun and Earth are eliminated, since they are the same for all. The principal sources of error, such as those due to the dis- tortion of the lens and changes of temperature, are radial, and may be determined, using both codrdinates of the lines in all the spectra. Changes in the differential refraction may be reduced, if desired, by turning the prism so that the spectra shall be horizontal, instead of vertical.