178 



NATURE 



\7aii. 4, 187: 



hand also. They are somewhat substantial structures, 

 formed of a stout wooden framework, covered with 

 weather-boarding and roofed with zinc and roofing-feh. 

 Each instrument has a separate hut. The transit huts 

 are ten feet square, with walls six feet high, and with the 

 shutter openings a little on one side of the centre, so as 

 to leave good room for mounting the clock, &c. The 

 altazimuth huts are planned on a nine-feet hexagon. 

 Their domes are hexagonal pyramids erected on circular 

 frames, which are grooved to run on six-inch rollers. These 

 rollers, six for each dome, are mounted on the wall-curbs. 

 One flap-back shutter gives sky view from the horizon to 

 the zenith. Each hut is made portable by being constructed 

 in sections which are connected together by bolts and nuts. 

 For the transit instruments massive Portland stone piers 

 and foundation slabs have been provided ; for the alt- 

 azimuths stone pier-caps only will be sent out, leaviny 

 the piers to be provided on the spot. Every part of each 

 observatory and every packing case has been numbered 

 and marked by stencilling, with a letter to denote the 

 station for which it is destined. 



These transit and altazimuth observatories, v/ith their 

 instruments and the primary clocks, are, with trifling ex- 

 ceptions, in perfect readiness for use. The equatorials are 

 generally ready, though their final completion has been 

 interrupted by the loan of portions of them to the observ- 

 ers of the recent solar eclipse. The telescopes will be 

 supplied with the Astronomer Royal's prismatic eye-piece 

 for correction of atmospheric dispersion, which will neces- 

 sarily be considerable at the low altitudes at which some 

 of the contact observations must be made. The equatorial 

 observatories are not yet constructed ; the plans for them 

 are under consideration as we write. The four-inch tele- 

 scopes, some second-class clocks for use with tlie alt- 

 azimuths and equatorials, and the small accessories, have 

 also to be provided. 



It is early to speak of the personnel of the various 

 observing expeditions. Officers of the army and navy 

 will proljably compose a large proportion of the observing 

 corps. Several gentlemen of the Royal Artillery have 

 already commenced practice at Greenwich with the time 

 and position instruments ; but, with the object of forming 

 a more accessible school of observation for them, a tem- 

 porary observatory has been fitted up near to their head- 

 quarters at ^^^oolwich. 



Photography was not included in the Astronomer 

 Royal's original plans. But from the time that his prepara- 

 tions were first mooted, the probable advantages of photo- 

 heliometry of the planet during transit were strongly 

 insisted upon. The plans for photography were advanced 

 from photographic quarters ; astronomers of the exact 

 class who were not photographers were somewhat scep- 

 tical at the outset concerning its accui'acy. They anti- 

 cipated that uncertainties would attach to the photographic 

 measurements ; in the first place from optical distortion 

 ofthe image formed by the camera-telescope ; in the second 

 place, from mechanical distortion produced by unequal 

 shrinkage of the collodion film, which must receive its 

 impression in the wet state, whereas the measurements 

 must be taken when it is dry ; and in the third place, it 

 appeared doubtful whether sufficiently accurate scale 

 measurements could be secured to make the results 

 equally reliable with those to be obtained from eye obser- 



vation of the contacts. No method of secondary accu- 

 racy could be tolerated, since the received value of the 

 solar parallax (8"'95) is probably much less than i per 

 cent, in error. It is considered that an eye- observation of 

 contact, i.e. of formation or rupture of the " black drop," 

 can well be made with no greater error than 4 seconds 

 of time. As Venus moves over the sun at the rate 

 of about 2" in a minute of time, the 4 seconds corre- 

 spond to a displacement of o"'i2 of arc in the direction 

 of motion, or about uJiTij of ''""^ sun's diameter. Can the 

 measurements from a photograph, with all the above 

 noted chances of error, l^e relied upon for such small 

 quantities ? It is argued that they can. The probable 

 error of a single micrometric measurement of the photo- 

 graphic distance of the images of a double star is cited 

 by Mr. Asaph Hall* to be o"'i2, and Mr. De La Rue, who 

 is naturally the English referee in such matters, has no 

 hesitation in saying that the measurements from a solar 

 photograph may be depended upon, with all due precau- 

 tions, to the ybooo °f ^^ sun's diameter. He is of opinion 

 that the shrinkage of the collodion film takes place only 

 in the direction of its thickness, and he considers that if 

 any optical distortion exists, it may be determined, and 

 the correction for it found, by photographing a scale of 

 equal divisions upon different parts of a plate, and com- 

 paring micrometric measurements of the various images. 

 Upon this point he is about to make some crucial experi- 

 ments with a large scale erected upon the Pagoda at 

 Kew, and photographed from the Kew Observatory with 

 the image in all positions on the sensitive plate. Herr 

 Paschen is also investigating the matter on the part of the 

 German Commission, using for his test-scale a glass plate 

 divided into squares by diamond-ruled lines. Some pre- 

 liminary trials have convinced him that should it be im- 

 possible to get rid of distortion, it will yet be easy to 

 correct for it as accurately as may be desired. 



Although the thorough reliability of the photographic 

 method has not yet been satisfactorily established, the 

 doubts concerning it have been in part removed, and it 

 has appeared undesirable to neglect photography in the 

 face of the circumstance that it might be the means of 

 obtaining some useful record ofthe transit at stations where 

 from atmospheric causes the observations of contact may 

 be lost or vitiated. Moreover, as other nations had de- 

 cided to employ the photographic method, it seemed in- 

 cumbent upon Britain to work in harmony if not in actual 

 concert with them ; for although there has as yet been no 

 formal proposal for international co-operation, there have 

 been communications between the astronomical authorities 

 of the various countries concerned, which have prevented 

 the formation of very divergent plans. The Astronomer 

 Royal therefore laid the subject before theBoard of Visitors 

 of the Greenwich Observatory, at their meeting in June 

 last, and it was fully discussed by them. They resolved 

 that it was desirable to furnish all the English stations 

 chosen for eye observations with the necessary photogra- 

 phic appliances, and an application was shortly afterwards 

 addressed to the Treasury for a grant of 5,000/. to defray 

 the expenses of the additional equipment. The money 

 was granted, and the construction of the photo-helio- 

 graphs — five in number — was forthwith placed in Mr. 

 Dallmeyer's hands. These instruments will be of generally 



' Sillimtii's yoiininl, vol. cii,. p. 26, 



