An^. 15, 1872] 



NATUR. 



Z'^Z 



direction of the Astronomer Royal, Sir George B. Airy. The 

 Russian Government will supply their own parties with three 

 similar instruments ; and I am also having constructed one of my 

 own for this purpose and for future solar ohser^'ations. All these 

 instrument?, made precisely alike, will embody the results of our 

 experience gained during the last ten years in photoheliography 

 at the Kew Observatory whilst belonging to this Association. 

 One only of them, namely, the photoheliograph which has been 

 at work for some years at Wilna, is of somewhat older pattern ; 

 but how great an advance even this instrument is on the original 

 at Kew is proved by the delighrful definition of the most delicate 

 markings of the sun in the pictures which have reached this 

 country from Wilna. 



Hitherto sun-pictures have been taken on wet collodion ; but 

 a question has been raised whether it would not be better to use 

 dry plates. On this point M. Struve informs us that, in two 

 places — at Wilna, undjr the direction of Colonel Smysloff, and 

 at Bothkamp, in Ilolstein, under Dr. Vogel — they have perfectly 

 succeeded in taking instantaneous photographs of the sun with 

 dry plate;. 



As far, however, as my own experience has gone, I still 

 believe that the wet collodion is preferable to the dry for such 

 observations. 



Xow, with reference to contact observations — which, it must 

 be remembered, are by no means indispensable as far as photo- 

 graphy is concerned — it may be conceded that there will attach 

 to the record of the internal contact a certain amount of uncer- 

 tainty, although not so great as that which affects optical obser- 

 vation. Tlie jjhotography which first shows contact may possibly 

 not be that taken when the thread of light between Venus and 

 the sun's disc is first completed, but the first taken after it has 

 become thick enough to be shown on the plate ; and this thick- 

 ness is somewhat dependent on incidental circumstances — for 

 example, a haziness of the sky, which, although almost imper- 

 ceptible, yet diminishes the actinic brilliancy of the sun, and 

 might render the photographic image of the small extent of the 

 limb which is concerned in the phenomenon too faint for future 

 measurements. On the other hand, having a series of photo- 

 graphs of the sun with Venus on the disc, we can, with a suit- 

 able micrometer — such as I contrived for measuring the eclipse 

 pictures of 1S60, .and which, since then, has been iu continuous: 

 use in measuring the Kew solar photograms * — fix the position of 

 the centre of e.ach body with great precision. But the reduction 

 of the measured distances of the centre to their values in arc is 

 not without difficulty. Irradiation may possibly enlarge the 

 diameter of the sun in photographic pictures, and it may diminish 

 the size of the disc of a planet crossing the sun, as is the case 

 with eye-observations ; but if the images depicted are nearly of 

 the same size at all stations whose reiultsare to be included in 

 any set of discussions, then the rat'o of the diameters of Venus 

 and the sun will be the same in all the plates, and it will be safe 

 to assume that they are equally affected by irradiation. The 

 advantage which, therefore, will result by employing no less than 

 eight instruments precisely alike, as are those now being made 

 by Mr. Dallmeyer on the improved Kew model, is quite obvious. 

 If other forms of instruments, such as will hereafter be alluded 

 to, be used, it will be essential that a sufficient number of them 

 be employed in selected localities to give also connected sets for 

 discussion. 



To give some idea of the relative apparent magnitudes of the 

 sun and Venus, I may mention that at the epoch of the transit of 

 1S74 the solar disc would, in the Kew photoheliograph, have a 

 semi-diameter of 1965 'S thousandths of an inch, or nearly two 

 inches ; Venus a semi-diameter of 63-33 of these units ; and the 

 parallax of Venus referred to the sun would be represented by 

 4 7 85 such units, the maximum possible displacement being 957 

 units, or n e irly ^'jth of an inch. 



When the photographs have been secured, the micrometric 

 measurements which will have to be performed consist in the 

 determination of the sun's .semi-diameter in units of the scale of 

 the micrometer, the angle of position of the successive situations 

 of the planet on the d sc, as shown on the series of photographs, 

 and finally the distances of the centres of the planet and the sun. 

 These data determine absolutely the chord along which the transit 

 \ as been observed to within o"-i ; and an error of I " in the mea- 

 suremtnt would give an error of only o" '185 in the deduced,solar 



* In this micrometer, which is capable of giving radial distance.", angles of 

 position, and also rectangular co-ordinates, the accuracy of lini 

 ments does not depend on ttie doubtful results given by a long rL 



parallax. Moreover the epoch of each photographic record is 

 determinable with the utmost accuracy, the time of the exposure 

 being from Jj to -^ of a second, or even less. 



Now, although the truth of the foregoing remarks will be fully 

 admitted, it will yet be well to point out in this place the inherent 

 or supposed defects of the photographic method. These defects 

 may principally be comprised under the head of Possibility of 

 Distortion ; and the importance of an investigation into this 

 source of error will appear at once obvious in all cases where the 

 position of a definite point with reference to a system of co- 

 ordinates has to be determined from measured photographs, 

 especially in such a refined application of it as that which it will 

 have iu the determination of tlie solar parallax. 



The distortion of a photographic image, if such exist, may be 

 either extrinsic or intrinsic — that is, either optical or mechanical. 

 The instrumental apparatus for producing the image may produce 

 optical irregularities before it reaches the sertsitive plate ; or an 

 image optically correct may, by irregular contraction of the sensi- 

 tive film in the process of drying, and other incidents of the 

 process, present on the plate a faulty delineation. * 



In general, two ways present themselves for clearing observa- 

 tions from errors. Either methods may be devised for deter- 

 mining the numerical amount of every error from any source, or 

 by special contrivances th-; source of error may be contracted to 

 such insignificant limits that its effect in a special case is too 

 minute to exert any infiuence upon the result. Both these roads 

 have been followed in the inquiry into the optical distortion of 

 photographic images. 



As regards the first, let it be supposed that, as in the Kew in- 

 strument, the primary image is magnified by a system of lenses 

 before reaching the sensitive plate. The defects inherent to the 

 optical arrangement will clearly affect every photographic picture 

 produced by the same instrument ; and hence a method suggests 

 itself for determining absolutely the numerical effect of distortion 

 at every point of the field. Let us assume that the same object, 

 which may be a rod of unalterable and known length, be photo- 

 graphed in precisely the same manner in which celestial events 

 are photographically recorded, the object being at a considerable 

 distance ; it may successively be brought into all possible posi- 

 tions in the field of the photoheliograph, and the length of the 

 image on the photograph may be measured afterwards at leisure 

 by means of a micrometer. These lengths will change relatively 

 wherever distortion takes place ; but by laying down these vary- 

 ing lengths we shall obtain an optical distortion-map of the 

 particular instrument; and tables may be constiucted giving in 

 absolute numbers the corrections to be applied to meas'irements 

 of positions on account of the influence of optical distortion. In 

 this way the optical distortion of the combined object-glass and 

 secondary magnifier is ascertained. The chief source of distor- 

 tion, if such exist, will be in the secondary maenifier ; and in 

 order to ascertain its amount, a reticule of lines drawn at equal 

 distances upon glass may (as has been done recently by Pascheii 

 and Dallmeyer) be placed in the common focus of the object- 

 glass .and secondary magnifier. The required data are then im- 

 mediately given by the measurement of the resulting pictures of 

 the parallelograms on the reticule. Mr. Dallmeyer has ascer- 

 tained in this manner that no sensible distortion exists in the 

 secondary magnifier constructed by him. The truth of the prin- 

 ciple being granted, it was applied to a preliminary seiies for 

 finding the distortion which affects the Kew instrument, which is 

 not nearly so perfect as those more recently constructed ; and the 

 results were so far satisfactory that, instead of a single rod, a 

 proper scale, fifteen feet in length, representing a series of 

 rectangles distributed over half the radius of the field has been 

 erected ; and the process of absolutely determining the optical 

 distortion of the Kew photoheliograph is now in active progress, 

 and will be used for the new instruments to be employed in 

 observing the transits of Venus. 



The second method of dealing with optical distortion aims at 

 total exclusion of this source of error. It has been proposed by 

 American astronomers, who intend taking part in the coming 

 observations of the transit of Venus, to exclude the secondary 

 magnifier, and, in order to obtain an image of sufficient diameter, 

 to employ a lens of considerable focal length, say 40 ft. , which 

 would give an image as large as with the Kew phutoheliograpb — 



* It has been proposed, in order to obviate any possible alteration of the 

 sensitive surface, to use the Daguerreotype instead of the collodion process. 

 The former, however, is so little practiced, and. moreover, is so much more 

 troublesome, that it does not seem to be advisable to ?dopt it, especially as 

 the subsequent measurements would prestnt greater difficulties than occur 

 with collodion pictures. 



