NA TURE 



[July 9, 1908 



pupillary adjustment. For the former, the focussing is 

 done separately for each eye by rotation of the eye-pieces, 

 controlled by a scale for future setting. Although at first 

 this mav appear more inconvenient than the usual double 

 screw motion of both tubes, this is not found to be the 

 case during continued usage, and the makers are enabled 

 to introduce the very desirable feature of making the 

 prism cases quite dust and moisture proof, which is almost 

 impossible when sliding tubes are employed. As issued, 

 the magnifying power is 6, and the field of view about 7°. 

 With respect to the varying opinions as to the best arrange- 

 ment of the object-glasses for stereoscopic effect, Messrs. 

 Leitz have decided that the advantages of placing them 

 further apart than the pupillary distance are questionable, 

 and so the object-glasses are fitted at the same interval 

 as the eye-pieces. The binocular is made of a specially 

 strong light metal, the weight being only 12 oz. without 

 case. We can without hesitation speak very highly of the 

 optical performance of this instrument. The definition is 

 remarkably crisp, and the image very achromatic and quite 

 sharp up to the edge of the field of view. 



A GENER.^L index to the annual volumes, sixteen in 

 number, published by the Geological Survey of Canada 

 since 1884, has been compiled by Mr. Frank Nicolas and 

 issued at Ottawa by the Geological Survey. The index 

 runs to 1014 pages, and contains about 180,000 references. 

 It should prove of great service to investigators anxious 

 to refer expeditiously to the annual volumes of the 

 Canadian Survey. The preseni catalogue, combined with 

 the index previously published, and dealing with the publi- 

 cations from 1863 to 1884, forms a complete means of 

 reference to the English edition of the volumes issued by 

 the Geological Survey of Canada. 



OVR ASTRONOMICAL COLUMN. 



Radial Velocities of Ninety-nine Stars. — The largest 

 single contribution to line-of-sight work yet made appears 

 in No. 5, vol. xxvii., of the Astropliysical Journal (pp. 

 301-24, June). The results were obtained by Prof. 

 KCistnor and Dr. Zurhellen, at the Bonn Observatory, 

 during the years 1903—7, and include the provisionally 

 determined velocities of ninety-nine stars of the second 

 and third spectral types down to the fourth visual, or fifth 

 photographic, magnitude. 



A three-6o°-prIsm spectrograph bv Topfer, giving a well- 

 defined spect.-um between W 4150 and 4500, was employed, 

 the temperature being automatically controlled by electric 

 means ; at Hy the linear dispersion is such as to give 

 152 tenth-metres per millimetre. 



Although the present values for the radial velocities are 

 only provisional, it is expected that they will not be greatly 

 modified in the final definitive results. In addition to 

 fifteen previously known variable velocities, the ninety- 

 nine sets of results include those for three other stars, 

 5 Tauri, e Bootis, and /i PegasI, the radial velocities of 

 which are suspected to be variable. The comparison spec- 

 trum employed In eacli case was that of the iron arc, 

 Kayser's values of the wave-lengths being taken ; Row- 

 land's values were taken for the stellar lines. As the 

 observations Included some 7500 complete measures of 

 about forty-four different stellar lines, Prof. Kiistner 

 expects that their discussion will provide good exact values 

 for the relative wave-lengths of the latter, and also in- 

 dicate their dependence on the type. In discussing the 

 determination of the constant correction, due, first to the 

 absolute errors of the wave-lengths adopted, and, secondly, 

 to the personal and Instrumental errors, Prof. Kiistner 

 considers as invalid the control usually obtained from plates 

 exposed on the sun, moon, or larger planets. He believes 

 Ihat a source of light, of precisely known radial velocity 



NO. 2019, VOL. 78] 



and as similar as possible to the star, should be observed, 

 and suggests the employment of the brightest minor 

 planets or of Jupiter's satellites for this purpose. After 

 many experiments, and at Dr. Zurhellen 's suggestion, he 

 employed spectrograms of the bright isolated peaks seen 

 at the moon's terminator, and found the results to be satis- 

 factory. These indicate that a small negative correction of 

 about — i-o km. should be applied to the results now pub- 

 lished. Of the constant radial velocities 'determined, that 

 of 7) Cephei, —85.98 km., is the largest. 



The Observation of Coloured Stars. — In No. 4252 of 

 the Astronomischc Nachrichten (p. 57), Herr Osthoff dis- 

 cusses at some length the changes of the colour percep- 

 tion of the eye, and shows that these changes depend upon 

 the physiological condition of the observer as well as upon 

 the intensity of the colour of the observed object and upon 

 the instrument used. A table containing the results of 

 his own observations between January, 1S94, and 

 November, 1898, shows the variation of the difference 

 between his estimates of colour and the catalogue colour 

 of the stars observed ; other tables show the variation of 

 the eye's colour-perception for red and yellow stars re- 

 spectively, and it appears that the eye is more uncertain 

 in estimating the red than the yellow. The importance of 

 this fact in observing tlie magnitudes of coloured variable 

 stars is pointed out. There is some indication of a 

 periodical change In the individual eye, but the observa- 

 tions are not sufficiently numerous to establish this. 



Photometric OiiStRV.ATiONS of Eros. — During the period 

 September, 1907, to January, 1908, Dr. Paul Guthnick 

 made a number of photometric observations of Eros at the 

 Berlin Observatory, and now publishes and discusses the 

 results in No. 4249 of the Astronomische Nachrichten 

 (p. I, vol. clxxvill.). From his discussion he is unable to 

 establish wilh certainty the existence of any short-period 

 light-variation. On plotting the light-curve, taking into 

 account the phase-variations, and trying periods of 5-24h., 

 5-28h., and 5-32h., he obtained a negative result. It 

 appears certain that during the greater pari of the opposi- 

 tion any short-period variation was imperceiitible. 



The Photography of Very Faint Spectra. — The 

 expedient of slightly fogging plates on which it is pro- 

 posed to photograph faint objects is generally known, but 

 is apparently not so generally adopted. Having recently 

 employe(a this procedure very successfully in the photo- 

 graphy of faint spectra, Mr. R. W. Wood, of the Johns 

 Hopkins University, describes his method and results in 

 No. 5, vol. xxvii., of the Astropliysical Journal (p. 379, 

 June). The curve representing the action of light on a 

 sensitised plate is at its commencement flat, but after 

 reaching a certain point it begins to rise much more 

 rapidly: Mr. Wood's supplementary exposure carries the 

 darkening of the plate to this point, so that the radiations 

 he Is wisiiing to photograph commence their action at that 

 part of the curve where a given exposure is much- more 

 effective in producing density than if it were applied alone. 

 By a judicious use of the method he has succeeded In re- 

 ducing the exposure, necessary to produce a certain density, 

 by one-half. The preliminary exposure needed is very 

 small ; with a gas flame turned down until the yellow tip 

 was but 3 mm. or 4 mm. high, four seconds at a distance 

 of about two metres was found to be sufficient. 



/t'LV AND AUGUST METEORS. 



'TPHE meteoric season of July has again returned, bring- 

 ing witli it ail the interesting associations attached 

 to this period in previous years. Early Perseids will now 

 be occasionally seen with their rapid flights, and leaving 

 streaks upon their paths, but they will be directed from the 

 southern region of Cassiopeia instead of from the place 

 4S° + 57°, ^s 3t fhe maximum epoch on August 11-12. 

 Many long-pathed and slow-moving .Aquarids will also be 

 noticed from the point about 339°— 10°, and this display 

 ofenerally develops its richest features near the end of 

 July, on about the 2Sth to 30th. 



