88 



NA TURE 



[May 28, 1908 



added, the increase being necessitated by the rapid growth 

 of experimental or physiological psychology during the past 

 few years. The character of the work remains the same, 

 and has been sufficiently described in reviews of earlier 

 editions. The price of the present volume is thirteen 

 marks. 



OVR ASTRONOMICAL COLUMN. 



Daniel's Comet, 1907^. — A continuation of Herr H. H. 

 Ivritzinger's ephemeris for Daniel's comet appears in 

 No. 424s of the Astrononiische Nachrichten (p. 345, 

 April 29). From this ephemeris we see that the comet is 

 now apparently travelling very slowly through the con- 

 stellation Virgo, nearly parallel to, and some 15' south 

 of, the equator. Its positions on June i and 13 will be 

 o=i3h. 521m., 5=— 0° i4'-3, and o=i3h. 48-im., 

 5=— 0° 2o'-2, respectively. The comet is now a little 

 brighter than the twelfth magnitude, and crosses the 

 meridian about 9.30 p.m. 



Spectroscopic Binaries. — For some time past the 

 observers at the Dominion Observatory, Ottawa, have been 

 endeavouring to complete their set of observations of the 

 spectroscopic binary / Orionis by obtaining spectrograms 

 at a critical part of the velocity curve where it changes 

 its form rapidly. .Vccording to a note in No. 2, vol. ii., 

 of the Journal of the Royal .Astronomical Society of Canada 

 (p. 106, -March-April), the necessary spectrograms were 

 secured at the end of January. The following principal 

 elements have been calculated from measurements of 107 

 plates: — period = 29.i36 days, eccentricity = 0-75, longitude 

 of the apse 110°, projected length of semi-major axis = 

 29,680,000 km., and velocity of the system=+20-7 km. 



Elements for >J/ Orionis, of which the radial velocity has 

 a remarkably short period, have also been obtained ; they 

 are: — period = 2.5259 days, e = o-o63, to = 1.80°, and iisini = 

 5,103,000 km. 



Mr. Harper has recently discovered that the star 

 5 Herculis has a variable radial \"elocity with a fairlv wide 

 range ; this star has therefore been added to the observing 

 list at Ottawa. 



Recent Observations of Jl'piter. — Some interesting 

 observations of Jupiter were reported to the March meeting 

 of the British Astronomical .Association by the Rev. 

 T. E. R. Phillips, who directed attention to the fact that 

 the great south tropical disturbance w-as once more pass- 

 ing the great Red Spot. The dark material of the dis- 

 turbance was observed to be passing round the south side 

 of the Red Spot by the south temperate belt, leaving a 

 sharply defined oval in which the Red Spot lay ; the latter 

 feature was very difficult to see, and appeared at times to 

 be distorted and irregular in form, as though clouds were 

 passing over it. Since the beginning of the apparition the 

 disturbance has increased considerably in length, from 

 about 60° in September to 115° at the end of January. 

 The rotation period of the Red Spot was less during the 

 earlier part of the apparition, but the diminution was not 

 so marked as it was during the conjunction of 1906, when 

 it amounted to 14° between April and August. At each 

 conjunction of these two features, since the first appear- 

 ance of the south tropical disturbance in 1901, the Red 

 Spot has, for the time being, appeared to be pushed for- 

 ward (the Observatory, May, Xo. 396, p. 196). 



Double-star Observers. — In continuing his series of 

 articles on double-star observers in the May Observatory 

 (No. 396, p. 205), Mr. Lewis gives a very interesting 

 account of the work performed by Baron Dembowski 

 between 1852 and 187S. With a telescope of 5 inches aper- 

 ture, and fitted with neither position circle nor driving 

 clock, Dembowski commenced a revision of the brighter 

 pairs given in the " Mensurre Micrometricae," and did 

 most excellent work. -After mentioning the double-star 

 work of Schiaparelli and other observers, Mr. Lewis pro- 

 ceeds to the consideration of what he calls the third period 

 of double-star astronomy, which began when Burnham 

 submitted a catalogue of eighty-one new double stars to 

 the Royal -Astronomical Society in 1873 ; all these doubles 

 had been observed with a 6-inch refractor mounted in 



XO. 2013, VOL. 78] 



Burnham 's back yard. In 1900 this same observer pub- 

 lished a catalogue of 1290 doubles discovered by himself 

 between 1871 and 1S99. Mr. Lewis also gives a long list 

 of observers of this third period, and the sizes of the instru- 

 ments with which they worked. 



In the note on the relative accuracy of various double- 

 star workers, which appeared in these columns last week, 

 the mean value given in the last line should, obviously, 

 be o"o69S, and not o".698 as printed. 



The CoLoiR Filter and Isochromatic Plate in -Astro- 

 NO.MICAL Photography. — -An important paper by Mr. R. J. 

 Wallace, on the function of a colour filter and " isochro- 

 matic " plate in astronomical photography, appears as a 

 reprint from the Astrophysical Journal for March. Mr. 

 Wallace discusses at length the use of various filters and 

 stained plates, and shows, by reproductions of some of 

 his photographs taken at the Yerkes Observatory, the 

 immense gain in definition resulting from such use. 



In the course of his discussion Mr. Wallace also refers 

 to the results recently published, by Prof. Lowell in his 

 paper on the sharpening of celestial photographic images, 

 and states a number of points whereon he disagrees with 

 the conclusions arrived at by Prof. Lowell. 



A New -Astronomical Journ.^l. — The first number of a 

 new journal, printed in Japanese characters, has just 

 appeared under the title of the Astronomical Herald, and 

 is published by the -Astronomical Society of Japan. This 

 first number is dated -April, and contains, among other 

 articles, a note on sun-spots by Mr. S. Hirayama, and one 

 on ancient astronomy. 



ALTERNATE CURRENl MEASUREMENT. 



'X'HE ordinary dynamometer is as well adapted for direct 

 "^ as for alternate current measurements ; but while it 

 is generally regarded as the best available instrument for 

 alternate currents, its use with direct currents is almost 

 restricted to standard instruments of the ampere balance 

 type. This is due to the immensely greater sensitiveness 

 obtainable with direct current instruments constructed on 

 the heterostatic principle, as exemplified in galvanometers 

 with permanently magnetised needles or magnets. The 

 latter instruments, due originally to Kelvin and Maxwell, 

 and first developed commercially by Ayrton and Perry, 

 have been brought to a high state of perfection in recent 

 years, with the result that ordinary measurements on direct 

 current circuits are much more precise and satisfactory than 

 those on alternate current circuits. 



The only likely way at present of improving alternate 

 current instruments is to use iron-cored electromagnets to 

 increase the strength of the field acting on the moving 

 system. The well-known difficulties due to the varying 

 permeability, hysteresis, &c., of the iron cannot be over- 

 come, but may be entirely avoided by exciting the electro- 

 magnet in shunt. 



The e.xcitation of an electromagnet can be governed 

 either by controlling the current through the exciting coil 

 or by controlling the voltage applied to the winding. On 

 direct current circuits the two methods are identical owing 

 to the operation of Ohm's law, but with alternate currents 

 the two modes of control lead to widely different results 

 if the resistance of the winding is made small compared 

 with the impedance. With current control, the magnetism 

 produced depends on the properties of the core, but is 

 independent of the resistance of the winding. The reverse 

 is true of the voltage controlled magnet, for, if this is 

 suitably designed, the magnetic flux is connected with the 

 applied voltage by a strict mathematical law not dependent 

 on the physical properties of the core, except for a small 

 correction term due to the resistance of the winding. The 

 shunt magnet in another way contrasts sharply with the 

 series magnet. In each case the strength of the magnet 

 is increased by diminishing the air gap, but the smaller 

 this is made the more accurate the shunt magnet becomes 

 and the less accurate the series magnet. The field due to 

 the shunt magnet is not in phase with the applied voltagi'. 

 so that special means must be used to supply the moving 

 coil of the instrument with a suitable current if the deflec- 

 tion is to indicate truly the quantity to be tested. But 



