September 23, 1897] 



NA TURE 



495 



•d'Ornano, the place where the Japan expedition had been 

 prepared. The question then arose as to the location of 

 the permanent observatory, there being two available 

 buildings belonging to the Government which could be 

 utilised for this purpose — one at Malmaison and another 

 at Meudon. The latter was finally settled upon, and the 

 partial restoration of the old chateau in the grounds was 

 commenced on lines suitable for the work to be under- 

 taken. 



The most satisfactory manner in which the whole of 

 this undertaking has been completed, will have been 

 noticed by those who have had the opportunity of visit- 

 ing this charming spot. Those less fortunate may gather 

 a good idea from an examination of the excellent series 

 of heliogravures which form part of this volume. The 

 old chateau, with its spacious grounds and rooms, has 

 proved suitable in many ways for such a physical 

 observatory. The large instruments have been housed 

 in appropriate domes, while the rooms devoted to 

 researches of several kinds have proved most convenient. 



Before referring to the work accomplished, let us 

 briefly make a survey of the instrumental equipment. 

 The large equatorial consists of a twin-telescope with 

 apertures of o'83 and o'62 metres, the 

 latter being devoted to photography. 

 These objectives have practically the 

 same focal length, being i6"i6 and 

 1 5 "go metres respectively. They are 

 set up together on the same mounting, 

 and housed under the large dome 

 situated in the middle, but at the upper 

 part, of the main building. The dome 

 itself is i8"5o metres in diameter, and 

 is rotated by means of electricity, the 

 same motive power being also used for 

 elevating or depressing the observing 

 platform. Two other smaller domes of 

 7 50 metres diameter, situated in the 

 grounds, contain respectively a New- 

 tonian reflector of i metre aperture and 



been maintained without any sensible leakage. Among 

 the investigations carried on here may be mentioned the 

 determination of the densities and spectra of gases under 

 high pressures. 



Coming now to that portion dealing with solar 

 photography, this is of special interest in that the 

 methods and results of a long series of researches 

 are given in a somewhat detailed form. Solar photo- 

 graphy at Meudon has attained such a high state of 

 perfection at the present day, that this may be looked 

 upon as one of the chief fruits of the observatory ; in 

 fact, quite a new era in this branch of the subject 

 has dawned. 



To describe the steps which have led M. Janssen to 

 achieve such a high state of excellence in photographing 

 minute details would necessitate a longer account than 

 we can here afford ; we must restrict ourselves, therefore, 

 to the main lines on which success has followed so 

 quickly and so surely. 



Investigations on the optical properties of the material 

 to be used for the lens as regards photographic ab- 

 sorption, showed that the glass gave a maximum amount 

 of light very confined in the violet region of the spectrum 



30 metres aperture, 

 focal length of 



a refractor of o" 

 The former has ; 



metres and was made by the brothers 

 Henry, M. Gautier having undertaken 

 the mechanical parts. This reflector 

 is mounted somewhat after the English 

 system, and is, as Prof. Janssen states, 

 a "precieux instrument de voyage." The 

 solar photographic telescope is de- 

 scribed as being of fine optical perfec- Fi 

 tion, and is due to Prazmowski. This 

 instrument will, however, be referred to later on, so we 

 may pass on to those parts of the building set apart for 

 laboratory work. 



In restoring the old chateau, as many of the smaller 

 buildings about it were retained as were likely to prove j 

 serviceable on a future occasion. Such, for instance, 

 was the case with the old stables, which measured nearly ; 

 100 metres in length, and could be easily extended I 

 another 40 metres if thought desirable. These have ' 

 since been appropriated for a large physical laboratory, , 

 and the oak partitions have been preserved in their | 

 original positions, serving among other things as useful 

 supports for the long tubes, which are employed in the 

 investigation of gases under pressure. The accompany- 

 ing illustration (Fig. i) gives a good general view of this 

 long corridor, and shows how the sides of the horse- 

 boxes have been utilised ; three of the long metal 

 tubes can be seen resting on the partitions, and lying 

 along the corridor a considerable distance. These tubes ! 

 measure 60 metres in length, their diameters being about 

 o'05 metres. They are joined in sections of 6 metres, j 

 and so securely is this accomplished, that for more than i 

 six months a pressure of from 20 to 30 atmospheres has j 



NO. 1456. VOL. 56] 



I. — The Meudon Stables as an Asirophysical Laboratory. 



near the Fraunhofer line G. A study of the sensitiveness 

 of the photographic film was next undertaken, and a 

 film was finally produced which was most active for 

 those rays which M. Janssen desired to employ. The 

 objective was then made so that the images formed at 

 the focus were nearly exclusively composed of rays of 

 the same refrangibility ; they were also, as M. Janssen 

 states, extraordinarily sharp. The next point was to 

 produce as fine a grain as possible in the film, and 

 of suflficient delicacy to reproduce all the details of the 

 image exposed on it. The results of these experiments, 

 giving the actual proportions which were finally adopted, 

 are described at some length. It is interesting to note 

 that great stress is laid on the importance of rigorously 

 cleaning the plate in the first instance We read " cette 

 proprete est ici encore plus indispensable que pour les 

 photographies artistiques les plus soigndes." Other 

 points of difficulty successfully overcome were : con- 

 siderable magnification of the image to show the delicate 

 details of the solar surface, and to diminish the effects of 

 irradiation ; complete control of the shutter, to ensure 

 sufficient exposure and exemption from irradiation ; and, 

 lastly, means of equally exposing all parts of the image. 



