June 9, 1898] 



NATURE 



^35 



for 1897 has been found to be very small, amounting to 

 + o"-o68 + o'-i04 sin L.D. The colatitude of the transit circle 

 obtained from 800 stars in 1897 was 38^ 31' 2i"-69, differing by 

 -o"*2i from the adopted value. 



The mean error of the moon's tabular place (computed from 

 Hansen's lunar tables with Newcomb's corrections) is -0^*142 

 in R.A. and +o"-27 in N.P.D. deduced from 95 observations. 

 These are equivalent to an error of - i"-97 in longitude and 

 + o"*i6 in ecliptic north polar distance. 



The New Altazimuth. 



The axis of this instrument has been considerably stiffened, 

 and modifications in the friction rollers have been made to re- 

 lieve the weight of the instrument on its bearings. Changes 

 have also been made in the illumination of the field and micro- 

 scopes. In December last the instrument was brought into 

 working order ; but regular observations have only recently been 

 commenced, as the determination of division errors, and other 

 observations necessary to test the stability of the instrument, 

 occupied several months' work. 



The observations on the whole show satisfactory stability in 

 the msirument, the collimation, level, and azimuth being steady. 

 Long series of observations of the nadir point have been made 

 to test the stability of the microscopes and of the instrument 

 generally for zenith distance observations. Discordances were 

 found in the results given by the two circles, which, after a con- 

 siderable time had been spent, were traced to the wheel carrying 

 one of the sets of microscopes, which was found to have worked 

 loose. This was remedied recently, and the accordance in the 

 results from the two circles appears now to be satisfactory. But 

 (arge changes in the readings of the individual microscopes are 

 found on turning the instrument into different azimuths, which, 

 however, would not affect the observations, as the microscopes 

 come back to sensibly the same readings for the same azimuth. 

 As, however, this implies a displacement of the microscopes 

 relatively to the circles when the instrument is turned, Mr. Simins 

 IS considering whether the supports of the microscopes and 

 pivots can be stiffened. 



Thompson Equatorial. 



Photographic tests with the 26-inch object-glass, varying the 

 distance between the two lenses, show that the images were 

 never good when away from the centre of the field. The glasses 

 were, therefore, sent back to Sir Howard Grubb for alteration, 

 and have only just been returned. A few trial photographs 

 show that the "coma" is now corrected, but that a slight re- 

 figuring IS still required. This, we are told, is being now done 

 by Sir Howard Grubb at the observatory. 



The 30 inch Cassegrain, mounted on the other end of the 

 declination a.xis, has been employed for obtaining photographs 

 of the moon, star clusters, and star fields. These have all been 

 obtained at the secondary focus, the focal length of the mirror 

 being somewhat longer than that for which the tube was de- 

 signed, making it impracticable to take photographs with it at 

 the primary focus. Dr. Common proposes to supply another 

 mirror of the correct focal length, 1 1 feet 3 inches. 



The photographic spectroscope has been completed, and is 

 mounted at the back of the cell of the 30-inch mirror, but the 

 diagonal prism to reflect the rays from the Cassegrain telescope 

 into the colhmator has not yet been mounted and adjusted. 



Aslrographic Equatorial. 

 The following statement shows the progress made with the 

 photo-mapping of the heavens : — 



For the Catalogue 

 (Exposures 



6in., 3111., and 20s.) 

 147 

 118 



For the .Chart, . 

 (Exposure 40111.) 



363 



285 



Number of photographs taken 

 ,, successful plates 



,, field photographed 



successfully 283 



Total number of successful 



fields reported 1897 May 10 551 



Number of photographs, pre- 

 viouslyconsidered successful, 

 rejected during the year ... 6 



Total numberof successfulfields 



obtained to 1898 May 10 ... 828 



Number still to be taken ... 321 



814 



909 

 240 



An important but unsatisfactory discovery has been made by 

 an examination of all the plates on the shelves. This has 

 shown that 166 catalogue plates out of 909 — that is, nearly one- 

 fifth of the total number — and 90 chart plates out of 828 have 

 deteriorated owing, probably, to the effect of damp in the 

 building in which they have to be stored pending the comple- 

 tion of the new physical observatory. There is difficulty in 

 warming this. building adequately, and the books, as well as 

 the photographs stored there, have suffered from damp. The 

 film has, in some cases, left the glass, and in the others shows 

 signs of doing so. Of the 166 damaged catalogue plates, 57 

 have been completely measured, 23 partially measured, and 86 

 are not measured. 



The importance of making duplicates as soon as possible of 

 all negatives in such a work as this cannot be underrated. 

 Positives on glass of all the 90 damaged chart plates were taken, 

 and these are uninjured. 



Of the fields still required, 197 are within 10° of the Pole, 

 and no photographs of this part of the sky have yet been taken, 

 the work being purposely deferred till near the epoch 1900. It 

 is proposed to begin taking these now, and the settings of the 

 scales for the guiding stars are partly computed. 



The 2%i)uh Refractor. 



This instrument was in use for micrometric measurements 

 from 1897 May 11 to 1898 May 10, with the exception of about 

 seven weeks, from August 5 to September 23, when it was used 

 for photography, the crown lens being reversed. During the 

 year 273 double stars have been measured, each star being 

 measured on the average on two nights ; the distance between 

 the components of these stars is less than i" o in 156 cases, and 

 in 63 less than o"-5. 



From August 5 to September 25, 1897, the instrument was 

 used with the crown lens in the photographic position. During 

 this period no measurable images of 17 double stars were ob- 

 t.ained on dry collodion plates. The closest of these pairs 



52881 

 22723 

 2 2900 

 22799 



i493> VOL. 58] 



spectroscopic and Heliographic Observations. 

 No spectro.scopic observations have been made during the last 

 twelve months. 



With the Dallmeyer photo-heliograph photographs of the sun 

 have been secured oh 191 days, 355 of these being selected for 

 preservation, besides nine photographs with double images of 

 the sun for the determination of zero of position. With the 

 Thompson 9-inch photo-heliograph twenty-two photographs 

 were taken on twelve days. Photographs to supplement the 

 Greenwich series have been received from India and Mauritius 

 up to 1898 February 22. 



For the year 1897 Greenwich photographs have been selected 

 for measurement on 183 days, and photographs from India and 

 Mauritius (filling up the gaps in the series) on i8r days, making 

 a total of 364 days out of 365 on which photographs are avail- 

 able. The importance of utilising the clear sky of India and 

 Mauritius for obtaining the photographs can hardly be better 

 demonstrated than by the figures given above, which show that 

 on only one day out of the whole year a photograph record of 

 the sun's disc was not secured. 



There has been but little change in the mean daily spotted 

 area of the sun for the period covered by the report as compared 

 with the preceding one. The progress towards minimum has 

 shown itself rather in the increase of days when the sun was 

 wholly free from spots, than in the poverty of the displays of 

 spots on the days when the sun's surface was disturbed. 



It will be remembered that about the time of the recent eclipse 

 in January there were several, comparatively speaking, large 

 spots on the solar disc, considering that the minimum period was 

 so near at hand. 



The remark made regarding the deterioration of the astro- 

 graphic plates applies also to many of the solar photographs, an 

 examination having shown that some of those stored in the 

 new library and in the museum of the physical observatory, both 

 gelatine and wet collodion, have suffered from damp, spots of 

 mildew being found on the film, though much more frequently 

 the mildew is confined to the uncoated side of the glass. 



