140 



NATURE 



{June 9, 1887 



decided to adopt Auwers' proper motions throughout. Prepara- 

 tions have accordingly been made for reducing the observations 

 in the Ten- Year Catalogue to the epoch 1880, with Auwers' 

 proper motions wherever available. 



It has appeared doubtful whether the reading of the exterior 

 thermometer placed near the north wall of the transit-circle room 

 represents the true temperature of the external air as affecting 

 the refraction for the sun and other southern objects in the day- 

 time. A discussion of simultaneous readings of the exterior, 

 front court, and meteorological' standard thermometers, which is 

 being made by Mr. Thackeray, shows systematic differences be- 

 tween the first and last at the time of observation of the sun, the 

 mean monthly excess of the meteorological standard over the 

 exterior thermometer for the ten years 1877-86 ranging from 

 -f o°7 in December to -(-2°'i in May and August and -l-2°-6 in 

 September. The reading of the front court thermometer (which 

 is at a distance from any building) appears to agree closely with 

 that of the meteorological standard, and it has been adopted, 

 from the beginning of this year, in computing refractions for the 

 sun, moon, planets, and stars south of the zenith observed in the 

 daytime, the exterior thermometer being still used for northern 

 stars as probably representing better the temperature of the air 

 on the north side of the transit-circle. The systematic differ- 

 ences in thermometer readings have a sensible effect on the 

 position of the ecliptic as deduced from observations of the sun, 

 the discordance in the results between the summer and winter 

 solstices found when the reading of the exterior thermometer are 

 used being rendered insensible when corrections are applied to 

 reduce to the reading of the meteorological standard thermo- 

 meter. 



Altazimuth, — The total number of observations of various 

 kinds made in the twelve months ending 1887 May 20 is as fol- 

 lows, the observations of the moon having been as usual restricted 

 to the first and last quarters in each lunation : — 



Azimuths of the moon and stars ... 

 Azimuths of the azimuth mark ... 

 Azimuths of the collimating mark 

 Zenith distances of the moon 

 Zenith distances of the collimating mark 



356 

 208 

 242 

 181 

 240 



The altazimuth observations are completely reduced to 

 March 31, so as to exhibit errors of moon's tabular R.A., 

 N.P.D., longitude, and ecliptic N.P.D., and the manuscript 

 for the printer has been prepared to the same date. 



Eqtiatorials. — Various additions have been made to the 

 Lassell equatorial with a view to making it available for astro- 

 nomical photography and for general use, A delicate slow 

 motion in R. A. (with differential wheels) and a firm N.P.D. 

 clamping arm with fine motion in N.P.D. have been applied, 

 the steadiness and general usefulness of the telescope being 

 greatly increased by these additions. The Corbett 6|-inch 

 refractor has been mounted below the tube of the reflector and 

 parallel to it to serve as a directing telescope in taking photo- 

 graphs and also for observation of occasional phenomena. A 

 camera to take circular plates 8^ inches in diameter (giving a 

 field 1° 58' in diameter) has been mounted at the principal focus 

 of the Lassell mirror, and some trial photographs of the moon, 

 Procyon, Regulus, 7 Leonis, and Praesepe, have been taken. 



The construction of the new 28-inch refractor has been 

 delayed by difficulty in obtaining the disks of glass. Messrs. 

 Chance are engaged in removing a bunch of fine veins from the 

 flint glass disk, and have every hope of being able very shortly 

 to report the disk practically perfect ; and M. Feil's successor 

 has successfully moulded a crown disk from which he believes 

 that he has removed all defects. 



The south-east and Sheepshanks equatorials are in good order. 

 Some trouble has been experienced with the water-supply for 

 the driving clock of the former instrument, and an alteration in 

 the arrangements for maintaining the pressure has been made 

 at the Kent Waterworks, since which the working has been 

 found quite satisfactory. 



The Cooke 6-inch equatorial is being mounted in the south 

 ground for trial as to the practicability of using curved plates for 

 stellar photography and other questions which have been raised 

 at the Paris Conference on Astronomical Photography. 



III. Spectroscopic and Photographic Observations. ' 



For determination of the motions of stars in the line of sight, 

 206 measures have been made of the displacement of the F line 

 in the spectra of 26 stars, and 20 measures of the b lines in 8 



stars, besides comparisons with lines in the spectrum of the 

 moon made in the course of the night's observations of star 

 motions, or of the sky on the following morning, as a check on 

 the general accuracy of the results. The observations of Sirius 

 since the date of the last Report indicate that the apparent di-^- 

 placement of the F line («hich was originally towards the red 

 and subsequently towards the blue) is now insensible. The dis- 

 placement of the F line in the spectrum of Algol has been 

 measured as frequently as possible during the winter months, in 

 order to ascertain if any evidence could be obtained of rapid 

 orbital motion such as would result from the hypothesis of the 

 variability of Algol being caused by the transits of a large satel- 

 lite. A sufficient number of observations has not yet been 

 obtained to allow a definite conclusion to be formed, but as far 

 as the observations go there are indications of a variation of the 

 motion in the line of sight corresponding to orbital motion, 

 having the same period as that of the star's variability. 



A photographic corrector, consisting of a concave crown and 

 convex flint lens (in contact), placed about 30 inches within the 

 focus, has been applied to the telescope of the south-east 

 equatorial to correct the chromatic aberration of the object- 

 glass for the photographic rays without alteration of the focal 

 length. A Dallmeyer doublet (formerly used in the photohelio- 

 graph) has been employed to enlarge the primary image about 

 7^ times, so as to give on the photographic plate an image on a 

 scale of about 0*45 inch to one minute of arc, or 15 inches to the 

 sun's diameter. A number of trial photographs of Castor, 

 7 Virginis, Venus, Jupiter, and Saturn have been obtained. 

 The photographs of the double stars appear to be susceptible of 

 very accurate measurement, and several of the photographs of 

 Jupiter show the four satellites, the belts, and the red spot. A 

 photograph of 7 Virginis, showing the components widely 

 separated, has also been taken at the primary focus, the Dall- 

 meyer enlarging doublet having been removed. It is intended 

 also to use the photographic corrector with the Dallmeyer doublet 

 to obtain photographs on a large scale of sunspots, craters on the 

 moon, and other objects of small angular dimensions. The field 

 of view with the photographic corrector is necessarily very 

 restricted. 



For the year 1886, Greenwich photographs are available on 

 199 days, and photographs from India and Mauritius filling up 

 the gaps in the series on 164 days, making a total of 363 

 days out of the 365 on which photographs have been measured, 

 the record being thus practically complete for 1886. 



As regards the photographic reductions : — 



The Greenwich photographs have been measured in duplicate 

 as far as 1887 April 28, and the measures have been completely 

 reduced so as to exhibit heliographic longitudes and latitudes of 

 spots and areas of spots and faculse. 



The photographs from India and Mauritius have been received 

 from the Solar Physics Committee as far as March 10 and 

 February 20 respectively, and these have all been measured, and 

 the measures completely reduced. 



IV. Magnetical Observations. 



The observations have been continued on the same lines as 

 in former years, changes in the magnetic declination, horizontal 

 force, and vertical force being continuously recorded by photo- 

 graphy and the absolute values of magnetic declination, hori- 

 zontal force, and dip being determined from time to time 

 by eye-observation. Earth currents in two directions nearly 

 at right angles to each other are also photographically registered. 

 For these last the ordinates have hitherto been measured on an 

 arbitrary scale, and it appeared desirable to obtain the data for 

 expressing this in terms of the accepted electrical units. The 

 authorities of the Post Office Telegraphs have courteously 

 given every assistance in regard to the requisite electrical 

 measurements, and an electrical balance for measuring re- 

 sistance, a standard cell, and a galvanometer of the Post 

 Office pattern have been procured under their auspices. In 

 October last, Mr. H. R. Kempe, of the Post Office Telegraphs, 

 made some measures of the resistances of the earth current wires, 

 but the conditions were not then favourable for insulation. 

 Subsequently the wires were damaged in the snowstorm of 

 December 26-27 l^st, and were temporarily .(repaired on January 

 25. It is believed that they are now restored to their normal 

 condition, and arrangements are being made to obtain the value 

 of the difference of electric potential between the two earth- 

 plates on each line corresponding to a given length of ordinate 

 on the photographic register. An experimental set of measures 

 of resistance has been taken recently. 



