8 



NATURE 



{May 5, 1887 



Peters, Pujazon, Roberts, Steinheil, Tacchini, Thiele, 

 Vogel, Weiss, Winterhalter, and Wolf. 



The proceedings of the section were opened by M. Cornu 

 in a long speech which gave rise to a very lengthened 

 discussion. 



The three following resolutions were finally arrived 

 at:— 



(i) All the plates to be used should be prepared 

 according to an identical formula to be subsequently 

 determined. 



(2) A permanent control of these plates from a point of 

 view of their relative sensibility to the different radiations 

 shall be instituted. 



(3) The aplanatism and achromatism of the object- 

 glasses employed shall be calculated for the wave-lengths 

 near Fraunhofer's G. 



At the second meeting the last resolution was re-dis- 

 cussed, and another one was passed modifying it as 

 follows : — 



" The resolution adopted at 'the last meeting relating to 

 the aplanatism and achromatism 9f the object-glasses 

 shall be understood in the sense that the minimum focal 

 distance shall be that of a ray near G with a view to 

 obtain the maximum sensibility of the photographic 

 plates." 



An important letter was read from Mr. Vogel, the 

 Director of the Potsdam Observatory, relating to the 

 preparation of the plates and suggesting the construction 

 of a sensitometer. 



The Committee then passed on to consider the extent 

 of the photographic field. 



With regard to the question of the distortion of images 

 of stars away from the centre of the field, the Astronomer- 

 Royal gave the results of calculations which he had made 

 to determine the dimensions of the elliptic images of 

 stars at different distances from the centre of the field, 

 supposing that at the centre the image is reduced to a 

 point. 



Calling the axes of the ellipses a and b, the Astronomer- 

 Royal's results are as follows : — 



The results of these calculations agree very satisfactorily 

 with measurements actually made on some of the Brothers 

 Henry's star photographs. 



It was suggested that, by placing the centre of the plate 

 slightly inside the focus, one might be able by a sort of 

 compensation to diminish the distortions of the more 

 distant parts of the field, and thus to augment the usable 

 extent of field. 



Finally, the following resolution was passed : — 



"The object-glasses shall be constructed in such a 

 manner that the field to be measured shall extend at least 

 1° from the centre." 



The decision was almost unanimous, three only voting 

 against it. 



Dundr then raised the question of duplicating the 

 observations on the same or different plates. 



There was a unanimous feeling that, although more 

 than 10,000 plates would be required if four square 

 degrees of surface were agreed upon for the field, two 

 series of negatives must be obtained for the whole 

 heavens ; the plates being so arranged that the star at 

 the corner of one plate shall be at the centre of another. 



The work of the meeting was terminated by suggestin-^ 



the appointment of two special Commissions to consider 

 questions relating to the safe keeping and reproduction 

 of the negatives, and also to the determination of photo- 

 graphic magnitudes. 



We next come to the meetings of the Astronomical 

 Section, which met on April 20, 21, and 22. 



The first part of the programme of this section was 

 the examination of the methods and instrumental details 

 which will enable the orientation of the plates and the 

 value of the scale to be precisely determined. The 

 question discussed was whether the actual plates record- 

 ing the stars down to the fourteenth magnitude could be 

 used for the determination of fundamental positions. It 

 was ultimately determined to have two series of photo- 

 graphs, but no resolution was arrived at at the first 

 meeting. 



At the second meeting this matter was settled by the 

 following resolution, which was carried unanimously, with 

 one exception : — 



" Besides the negatives giving the stars down to the 

 fourteenth magnitude, another series should be made with 

 shorter exposures, to assure a greater precision in the 

 micrometrical measurement of the fundamental stars, 

 and render possible the construction of a catalogue." 



At the third meeting the President remarked that, as 

 the section had finished its deliberations, it was desirable 

 to arrive at final resolutions on the questions referred 

 to it. 



M. Auwers proposed the following resolution : — 



" The supplementary negatives destined for the con- 

 struction of the catalogue shall contain all the stars down 

 to the eleventh magnitude inclusive. The Executive Com- 

 mittee shall determine the steps to be taken to insure 

 that this condition is fulfilled." 



This was carried by 25 votes to 6 ; some members 

 being in favour of tenth magitude only. 



The second resolution, which was unanimously agreed 

 to, ran as follows : — 



" The photographic plates to be used in formation of 

 the catalogue shall be accompanied by all the data neces- 

 sary to obtain the orientation and the value of its scale ; 

 and as far as possible these data shall be written on the 

 plate itself. 



" Each plate of this kind shall show a well centred copy 

 of a system of cross-wires to insure the determination of 

 errors of the field, and to eliminate those which may be 

 produced by a subsequent deformation of the photo- 

 graphic film. 



" Further details of this nature shall be determined by 

 the Executive Committee." 



The following resolutions were next carried : — 



" In the negatives destined for the map the number of 

 cross-wires to be used in their control and reduction shall 

 be reduced to a minimum." 



" The tubes of the photographic instruments shall be 

 constructed of the metal most likely to give an invariable 

 focal plane, and shall carry a graduation for the 

 determination and regulation of the position of the 

 plate." 



" The Executive Committee shall choose the reference 

 stars to be used " 



" The question of the methods ot measurement and the 

 conversion of the numbers obtained in right ascension 

 and declination for the equinox of 1900 is left to the 

 Executive Committee." 



" That Committee shall first occupy itself with the study, 

 and methods of use, of measuring-instruments giving either 

 rectangular or Polar co-ordinates, and based upon the 

 simultaneous use of scales for the larger distances and 

 micrometer eye-pieces for scale subdivisions." 



