12 



NATURE 



{May 7, 1874 



be effected ; the photographic method, the heliometric 

 method, and the method of durations. We shall consider 

 these in order. 



I. The Photographic Mclhod.-U is easy to see that by 

 continuing during the transit to take photographs of the 

 sun, in which A'enus will be represented as a black spot, 

 these photographs may be so combined as to indicate 

 definitely the apparent path of Venus as seen at these two 

 stations. This method is looked forward to with much 

 interest, because it is the first time that photography has 

 been extensively employed in delicate astronomical mea- 

 surements. It is not generally known how extremely 

 accurate a means of observation photography is. We 

 owe much to Mr. De la Rue, whose success in the appli- 

 cation of photography to astronomy has been unequalled, 

 for having given us a most clear account of what has 

 been done in this way.* The method has been employed 

 in America to measure the distances between double stars. 

 The double star is photographed and the distance is after- 

 wards measured as accurately as possible. Prof Bond 

 finds that the probable error of such a measurement is 

 d'-oyz or i of the probable error of a similar measure 

 made with a filar micrometer as estimated by Struve. 

 Photographic pictures of the sun were for many years 

 daily taken at Kew, and it was found that an extremely 

 accurate measure of the sun's diameter could thus be 

 made. Ifthelensof a common telescope were used to 

 produce an image of the sun upon the sensitive plate 

 the picture would be too small for accurate measurement. 

 Hence a special instrument called a photoheliograph must 

 be devised to give an enlarged picture upon the sensitive 

 plate. Two perfectly distinct kinds of instruments are 

 to be used for this purpose, the one English, the other 

 American. Mr. Dallmeyer has, under the superintendence 

 of Mr. De la Rue, constructed photoheliographs for the 

 English and Russian expeditions. In these instruments 

 the image of the sun produced in the focus of an ordi- 

 nary telescope is enlarged by a special arrangement so as 

 to give a picture of the sun about four inches in diameter. 

 This instrument, based upon the principle of the Kew 

 photoheliograph, is veiy perfect in its results and con- 

 venient in actual practice. It is mounted equatoriaUy so 

 as to follow the motion of the sun. The sensitive plate, 

 which is prepared in an adjoining room, can be readily 

 inserted and exposed. The intensity of direct so'ar light 

 is so great that special means are necessary to give a 

 short enough exposure. Before a photograph is taken a 

 sliding shutter in the interior of the instrument cuts off 

 all light from the sensitive plate. This shutter is held in 

 its place by a cotton thread. So soon as this thread is 

 cut, a strong spring draws down the shutter, in which 

 is a slit about /oth of an inch wide. The time taken by 

 this slit to pass over any part of the sun's image is the 

 whole interval required for an exposure. 



The other method of obtaining a large picture of the 

 sun is by employing a lens of great focal length. This 

 method was originally proposed by Mr. Rutherford, of 

 New York, and will be employed by the Americans, and 

 also by Lord Lindsay in his observations at the Mau- 

 ritius. The focal length of the lens is forty feet. But a 

 telescope of such dimensions could not be conveniently 

 mounted in the ordinary way. To overcome this, a side- 

 rostat similar to the one originally constructed by M. 

 Foucault for the Observatory of Paris is employed. This 

 instrument consists of a plane mirror so mounted as to 

 send the sun's rays always in the same horizontal direc-- 

 tion. In the path of these rays, and close to the side- 

 rostat the lens is placed, and at a distance of forty feet an 

 image of the sun about four inches in diameter is 

 produced. At this place a window is arranged in the 

 photographer's hut, and by means of this arrangement the 

 photographer need never leave his dark room. After pre- 



* Address to the Mathematical and Physical Section of the British Asso- 

 ciation, Brighton, 1872. 



paring a plate he places it in position at the window ; 

 when exposure has been made he may remove the plate 

 and develop it. 



Considerable advantage is likely to accrue to the 

 employment of dry plates, which will diminish the 

 labour of the photographer. Researches upon this 

 matter have been undertaken by Prof. Vogel, in Holstein, 

 Col. Smysloff, at Wilna, and by Capt. Abney, at Chatham. 

 The employment of a dry process prevents all danger 

 from the shrinking of the collodion- film. Herr Paschen * 

 and Mr. Ue la Rue have made experiments upon this 

 point. The latter gentleman finds that all shrinkages 

 take place in the thickness of the film, so that the mea- 

 surements would not be affected by it But the more 

 convenient dry plate process is undoubtedly safer. Judg- 

 ing from the data furnished by Mr. De la Rue, this pho- 

 tographic method will give results of the utmost value. 



I I. The Heliometric Method. — The exact measurement 

 of the distances of the edges of Venus from opposite 

 edges of the sun would enable us easily to determine what 

 is required, viz., the least distance between the centres 

 of the sun and planet. But the ordinary astronomical 

 means are useless in measurements of this magnitude. 

 To obviate this, a special instrument, called a heliometer, 

 will be employed by the Germans and Russians, and by 

 Lord Lindsay. This instrument was origmally invented 

 for measuring the diameter of the sun. The object-glass 

 of a common telescope is divided so as to form two semi- 

 circles. A screw adjustment allows us to slip one-half 

 of the lens past the other one along their line of junction ; 

 a fine scale measures this displacement. When the two 

 halves of this object glass are relatively displaced, two 

 images of the sun are seen overlapping. The distance 

 between the two images is proportional to the relative 



i displ cement of the two halves of the object-glass. This 

 instrument has been brought to a state of great perfection 

 by Mr. Repshold, of Hamburg. It is a very troublesome 

 instrument to manipulate, and the corrections due to the 

 influence of temperature are extremely difficult to apply. 

 Yet with great care there is little doubt that very accurate 

 measurements can be made. The nature of the measure- 

 ments required to obtain the distance between the centres 

 of Venus and the sun will readily be understood. The 

 method has been most ably discussed by Lord Lindsay 

 and Mr. Gill in the Monthly Notices of the R.A.S., 

 November 1872. At the same time it is difficult to con- 

 ceive that this direct method will give results of equal 

 value with the methods hereafter described. In fact, an 

 opposition of Mars would be expected to give equally 

 good results ; for the distance of Mars from a fixed star 

 can be more accurately observed with a micrometer than 

 the distance between the centres of Venus and the sun ; 

 and a larger number of observations could be made. 



III. 7 lie Metliod of Duration. — The third method of 

 determining the least distance between the centres of the 

 sun and Venus is less direct than either of the preceding 

 methods ; but it has stood the test of a previous trial, 

 and we cannot say but that it will be more satisfactory 

 than the other methods in the coming transit. The 

 method of duration closely resembles the method origi- 

 nally proposed by Halley. The duration of the transit, 

 as viewed from two distinct stations, is accurately deter- 

 mined. But the difference in this duration is affected by 

 choosing stations upon a different system. Nevertheless 

 this method is frequently called Halley's method. His 

 method consisted in choosing two stations, so that during 

 the transit the one should be moving eastward and the 

 other westward. It is further essential for success tliat 

 Venus should pass nearly along the diameter of the sun. 

 In the method employed last century, the two stations 

 were chosen — the one far north, and the other far south. 

 On referring to Fig. 1 1 it will be seen that in each case 

 Venus appears to pass along a chord of the sun. But in 



* Astronoittiscke Ntichrkhtcn, 1S72, Ixxix. i6r. 



