August 10, 1906.] 



science: 



183 



Belopolsky discovered, in 1896, that the 

 fainter, preceding component of Castor, a^ 

 Geminorum, was a spectroscopic binary, hav- 

 ing a period of about 2.9 days. In the June 

 number of the Astrophysical Journal Dr. 

 Heber D. Curtis gives an elaborate investiga- 

 tion of this system, which has been made at 

 the Lick Observatory. The refined results 

 now obtained amply confirm the general 

 conclusions of Belopolsky. The final period 

 found at the Lick Observatory is 2.928285 



Curtis has also discovered that the brighter, 

 following component, a^ is a spectroscopic 

 binary having a period of 9.218816 days. 

 Ignoring the faint and distant companion 

 which also exists. Castor thus becomes one of 

 the most wonderful stellar systems known. 

 While the elements of the spectroscopic sys- 

 tems thus revealed are well known, less can be 

 said of the visual components of Castor them- 

 selves. These have been observed since the 

 time of the elder Herschel, but their motion 

 is slow, and Burnham considers that the prob- 

 lem of finding the elements is indeterminate 

 at present, and likely to remain so for a cen- 

 tury. Different determinations of the period 

 have varied from little more than 200 years 

 to more than 1,000 years. 



An interesting phase of this quadruple sys- 

 tem lies in the fact that while the orbits of 

 the components of (\ are nearly circular, those 

 of a^ are very elliptical. The change in the 

 mutual attractions of the components of so 

 eccentric a system as that of a., must be great, 

 and in other cases this has been found to be 

 associated with variability in the light. On 

 this account Mr. Curtis made some photo- 

 metric determinations of' the light of a^, but 

 no proof of variability was found. This need 

 not cause surprise, however, since the physical 

 condition of a star must determine whether 

 such increased attractions would show them- 

 selves in a change of luminosity. 



REFLECTING TELESCOPES OF SHORT FOCUS. 



H. C. VoGEL, director of the Astrophysical 

 Observatory at Potsdam, calls attention, in a 

 communication to the Kgl. Akademie der 

 Wissenschaften, Berlin, to the possibilities of 



short-focus reflecting telescopes. A transla- 

 tion of this article appears in the June num- 

 ber of the Astrophysical Journal. The com- 

 petition, which has always existed between 

 reflecting and refracting telescopes, is still 

 keen, but at the present moment the reflector 

 seems to occupy the center of the stage. Each 

 form of instrument has certain limitations 

 which are inherent, and each has distinct ad- 

 vantages over the other for certain purposes. 

 A good reflector brings all the colors perfectly 

 to the same focus, while the best refractor is 

 not really achromatic. Also, the loss of light 

 by reflection is in general much less than that 

 by absorption. The result is that for nearly 

 all objects the action of a reflector is much 

 more rapid than that of a refractor of equal 

 size. The circles of chromatic aberration 

 may also cause a distortion of the form of the 

 image of certain objects, such as the Orion 

 nebula, when photographed with a refractor. 

 On the other hand, the field of good definition 

 is very small, in the case of the reflector. The 

 images are perfect only at the center, and 

 sufficiently good for use only over a region 

 half a degree in diameter, or even less. Cer- 

 tain forms of the refractor, however, give 

 good definition over a field ten, or even twenty, 

 degrees in diameter. It is evident, therefore, 

 that where a large field is required, as in 

 making a photographic map of the sky, the 

 refractor must be used. Over a small field, 

 however, the reflector can photograph objects 

 of such extreme faintness that they lie quite 

 beyond the reach of the largest refractor. 



Schwarzschild has shown that an extension 

 of the field of the reflecting telescope may be 

 obtained by the use of a second mirror. It is 

 possible that the diameter of the field may be 

 extended to several degrees in this way. Prac- 

 tically, this has not yet been done, but its 

 successful accomplishment would mark an 

 epoch in the history of the telescope. The 

 power of the reflector evidently increases with 

 the decrease of its focal length. Schaeberle 

 has recently made some experiments to show 

 how far this can be carried. He constructed 

 a thirteen-inch reflector of twenty-inch focus, 

 with which he was able to photograph stars of 

 the seventeenth magnitude with an exposure 



