34 



NATURE 



[June lo, 1897 



patches on her surface will feel satisfied that this well-known 

 observer has not yet detected such tracings as have been put 

 before us by Lowell. Venus was frequently observed by him 

 with the 12-inch refractor of the Mount Hamilton Observatory 

 during the years 1888-95, but as he says he " never could (with 

 butone exception) satisfactorily seethe markings. Vague indefinite 

 spots were often visible, but it was not possible to see them well 

 enough to identify them for rotational purposes." With these 

 facts before us, it is not then surprising that the observed periods 

 ol the planet should vary from twenty-three or twenty-four hours 

 to 225 days. The exceptional case of good seeing mentioned above 

 was •' when the air was thick with smoke and dust. ... I was 

 struck with the remarkably perfect definition. There was not 

 the slightest tremor. The markings on the surface of the planet 

 were distinctly seen, though they were difficult and very 

 delicate." The drawing which accompanies Prof. Barnard's 

 description shows the crescent of Venus with four large hazy 

 patches very much foreshortened in their position near the limb. 



To continue his series of measurements of the diameters of all 

 the planets with the 36-inch, Prof Barnard, in May, June and 

 July of 1895, undertook that of Venus. The mean of all his 

 measures reduced to unit distance gave a diameter of I7"'397> 

 corresponding to an actual diameter of 7826 miles. 



This value seems to be very satisfactory when compared with 

 the mean of all previous determinations, as will be seen in the 

 following table. 



Hartwig ... Breslau heliometer ... ... ... 17 '67 



,, ... Reduction of Oxford measures ... I7"582 



,, ... Double image observations by Kaiser 17409 



,, ... Nine measures in Bahia-Blanca ... 17406 



Peter ... Two measures in Bahia-Blanca ... i7-2i6 



Kiistner ... Two measures in Punta Arenas ... 17 "3 12 



Auwers ... Measures during transit i6-8oi 



Ambronn... Gotlingen heliometer ... ... \']"]\\ 



Barnard... 36-inch Lick refractor!.. 



Mean 17 '389 

 ••• 17-397 



Nebul/e unrecorded in Catalogues. — In the current 

 number of the Observatory, Dr. Roberts gives a list of several 

 nebulffi which have not found a place in catalogues, but which 

 have been recorded on the plates used in his photographic 

 survey. These photographs were taken with his fine 20-inch 

 reflector at Crowborough, and a comparison between these and 

 the recorded places of nebuk^ in the " New General Catalogue," 

 and the " Index Catalogue," by Dr. Dreyer, has been fruitful of 

 many discoveries. Of the seventeen new nebulse, we extract 

 the following description of the largest : — 



Region of ^I. 157 Trianguli, N.G.C. 672; Nova, R.A. 

 ih. 39m. 39s. N.P.D. 63° 22''3. — It is nearly as large and 

 prominent as ^I. 157, and distant from centre to centre 

 8' only ; nucleus consists of six faint stellar condensations form- 

 ing a straight line in the direction south, following to north 

 preceding, and there are six or seven very faint condensations 

 of nebulosity near the preceding margin ; 15th mag. star on the 

 north preceding margin, and a i6th mag. star at the south 

 following end of the nucleus, 1896 November 29. 



It is remarkable that this object should have escaped detection 

 by the many keen-eyed observers who have examined the nebula 

 yl. 157, which is only four minutes of arc distance from it ; and 

 it appears to me that we are justified by the evidence in inferring 

 that this nebula has come into the state of visibility during the 

 past half-century. Lord Rosse, in 1896, made several observations 

 of the nebula adjoining, but does not refer to this one. It is re- 

 markable also that the nuclei of the two nebulte are straight lines 

 of faint nebulous stars immersed in nebulosity, and they are so 

 clearly depicted on the photograph that I think they should be 

 visible to the eye by the aid of telescopic power. 



Dr. Roberts finds further, by examining his negatives, that two 

 classes of stars, which he terms "faint" and " small,"' attract 

 notice. The former have small bright nuclei surrounded by 

 nebulosity, and are quite distinct from the latter, which appear 

 as small round spots without a nucleus. These, he states, 

 would, if they were classified, come under the heading, " small 

 circular nebuls; with small bright stellar nuclei." 



' Harvard College Observatory Zone Observations. — 

 Volume xxxvi. of the Annals of the Astronomical Observatory of 

 Harvard College contains the journal of the zone observations of 

 stars between 49° 50' and 55° 10' of North Declination. These 



NO. 1 44 I, VOL. 56] 



observations were made with the meridian circle during the ten 

 years 1875 to 1885 by Mr. William Rogers, under the direction 

 of the silccessive directors Joseph Winlock and E. C. Pickering. 

 The present volume completes the journal begun in volume 

 XXXV., and in this review all doubtful cases have been re- 

 examined. 



THE ROYAL OBSERVATORY, GREENWICH. 

 T^HE Astronomer Royal presented his annual report on 

 Saturday last to the Board of Visitors of the Royal 

 Observatory, Greenwich. Among the numerous guests were 

 many astronomers and men of science, who inspected the 

 buildings and instruments, especially those which have been 

 erected since the visitation last year, namely, the Thompson 

 equatorial and the new altazimuth. The following extracts from 

 the report contain a brief resume of the year's work. 



Buildings. 



The building of the north wing and central dome of the 

 Physical Observatory was finished in September 1896, with the 

 exception of the vane on the central dome, which was completed 

 last March, 



An observing floor and gallery have quite recently been fitted 

 up in the dome to facilitate work with the new Thompson 

 equatorial, now mounted there. The completion of the Physical 

 Observatory by the building of the east and west wings has been 

 further delayed, though provision was made for commencing the 

 work in the last financial year. 



The Transit Circle. 



With regard to this instrument, it has been found that the 

 apparent correction for discordance between the nadir observa- 

 tions and stars observed by reflexion has been gradually increas- 

 ing yearly, the diff"erence for the present year being -o"'44, 

 the greatest negative value recorded since 1888. 



The increase in this discordance in 1896, following on the 

 systematically negative values since 1891, led to a re-examination 

 of the screws of the microscope-micrometers, of the screw of the 

 telescope-micrometer, and of the errors of those divisions of the 

 circle which are used in observations of the nadir, with a view 

 to the discovery of the source of this discordance. 



The microscope-micrometers showed signs of wear, but the 

 reversal of three of the screws has successfully eliminated the 

 effect of wear from the mean of the six microscopes. 



The Netu Altazimuth. 

 This instrument was erected in May 1896, but it was not 

 practicable to make observations with it till the completion of 

 the observing floor in September. It was then found that there 

 were serious discordances in the readings of the circles under the 

 different micro.scopes, depending on the direction in which the 

 instrument was last turned. Experiments indicated flexure in 

 the axis, which has now been corrected by stiffening the axis 

 by means of a strong diaphragm of special form fitted in the 

 central part of the axis. The friction-rollers for taking the 

 weight of the instrument have also been modified, the position 

 being changed to bring them close to the pivots, and a system of 

 levers has been substituted for springs. These changes reduced 

 the discordances greatly in amount. Quite recently Mr. Simms 

 has discovered a cause of error, arising from a tendency in the 

 pivots to act as a screw, a longitudinal force being thus intro- 

 duced, its direction depending on the direction in which the 

 telescope is turned. This force had the effect of slightly moving 

 the iron standards carrying the bearings and the microscopes, 

 thus changing the position of the microscopes relatively to the 

 graduated circles. This action of the pivois was lound to arise 

 from the method adopted in grinding them of giving a helical 

 twist to the grinder, and it was cured by a few circular turns of 

 the same tool. 



The Thompson Equatorial. 



This new instrument, presented by Sir Henry Thompson, 

 forms a handsome addition to the Observatory, and it has 

 been mounted in the Physical Observatory under the Lassell 

 Dome. Its erection there was commenced early in November, 

 but it was not ready for use till April, and there are still certain 

 accessories which have to be supplied. The adjustment of the 

 polar axis and of the 26-inch object-glass were at once taken in 



