DXOEMBEB 11, 1908] 



SCIENCE 



849 



system about the center of mass of which the 

 eclipsing pair revolve in a nearly circular orbit 

 with a radius of not less than 89,000,000 km., 

 a period of 1.899 years and an epoch of minimum 

 radial velocity at the date, 1901.85. 



The variations observed in the eclipse period 

 of this star, as well as those suspected in the 

 eccentricity, are possibly largely due to perturba- 

 tions arising in such a system. 



As a consequence of the orbital motion of the 

 center of mass of the eclipsing pair a variation in 

 the time of light minimum with a range of ten 

 minutes and a period of 1.899 years should be 

 shown by photometric observations. 

 Aohromatio and Apoohromatic. Comparative Tests. 

 Preliminary Communication: E. D. Roe, Je. 

 To test them side by side, two visual telescope 

 objectives, each of the two-lens type, an achro- 

 matic and an apochromatic of 2J-inch and 2g-inch 

 aperture and 44 inches focal length, were ordered 

 by the writer in March of Mr. Lundin, of the 

 Clark Corporation, and of Steinheil Sohne, of 

 Munich, respectively. Hartmann's Foucault knife- 

 edge test as described in the Asirophysical Journal 

 for May was tried by Dr. Saunders and the writer 

 on Mr. Lundin's objective, both visually and pho- 

 tographically. Color screens were used for testing 

 chromatic aberration. Four photographic plates 

 of the objective were secured. The objective 

 showed high excellence under these tests. For the 

 laboratory manipulations and appliances involved 

 the writer is greatly indebted to his colleague. 

 Dr. Saunders. Mounted on a 6J inch Clark equa- 

 torial with clock-work, the writer has tested the 

 objective on double stars with splendid results. 

 As the Steinheil objective was only recently re- 

 ceived, the investigation is as yet unfortunately 

 incomplete. Preliminary examination, however, 

 raises the expectation that this objective will 

 accomplish what its maker intended it should. 

 On a quick Visual Method of redetermining the 

 Focus of a large Visual Refractor when used 

 for Photography with a Color-screen: E. E. 

 Babnabd. 



Such a large telescope as the 40-inch refractor 

 of the Yerkes Observatory changes its focus 

 largely from changes of temperature. But the 

 photographic focus of such an instrument, where 

 a color-screen is used, having once been deter- 

 mined, can at any time be redetermined accu- 

 rately, regardless of temperature changes. 



The following is the method: Find the photo- 

 graphic focus by the usual method of exposure 

 on the stars. Insert in an adapter on the plate- 



holder a high-power eyepiece with graduated scale 

 on the tube. With the plate-holder set at tha 

 scale reading giving the best result, focus care- 

 fully with the eyepiece on a star, and record the 

 reading on the scale of the eyepiece. At any other 

 time, disregarding the temperature, etc., the plate- 

 holder is set at the original scale reading for the 

 best focus, the eyepiece is inserted and the visual 

 focus read oif. If it is so many millimeters 

 shorter or longer than the original visual reading, 

 the plate-carrier must be moved in or out by that 

 extent. The plate will then be in the beat focus 

 for the moment. If a spider line is inserted in 

 the eyepiece and is brought into the focal plane 

 with the star, then any other observer can at any 

 time determine the focus in a minute's time by 

 bringing a star and the wires into focus and 

 making the proper connections. 

 On the Focal Changes in Nova Persei and on the 

 Focus of some of the Wolf-Rayet Stars: E. E.' 

 Baenaed. 



In A. N. No. 4232, Dr. J. Hartmann finds that 

 the spectrum of Nova Persei changed to the neb- 

 ular condition in the fall of 1902. In 1906 it had 

 again changed and was similar to that of the 

 Wolf-Eayet stars. Observations in the rather long 

 interval were lacking to show when the change 

 from the nebular spectrum occurred. 



Observations of the focus of this star were made 

 at the Yerkes Observatory with the 40-inch in 

 1901, 1902 and 1903. They show a change in 

 accordance with Dr. Hartmann's spectroscopic 

 observations. 



The focus rather suddenly became nebular about 

 the first of October, 1902, when it was a quarter 

 of an inch longer than for a star. This lasted 

 until the middle of November of the same year, 

 when it slowly returned to the stellar focus, be- 

 coming stellar about February, 1903. The last of 

 these focal measures was made September 28, 

 1903. The result was — 0.03 inch. This value 

 was so small that the sign was not considered real. 

 But for the fact that Dr. Hartmann found the 

 Nova had finally reached a condition similar to 

 that of the Wolf-Rayet stars, it may have a 

 stronger significance than I supposed, for I find 

 that those of the Wolf-Rayet stars I have ex- 

 amined, with the exception of one case, have their 

 focus slightly shorter than for an ordinary star. 

 The focal measures, therefore, verify Dr. Hart- 

 mann's results and show, furthermore, just when 

 the change in the spectrum of the star occurred. 

 Approximate Ephemerides of Fixed Stars: G. C. 



COMSTOCK. 



