4 KEPORT 1872. 



solar parallax. Moreover the epoch of each photographic record is determinable 

 with the utmost accuracy, the time of the exposure being from j'j- to y^o 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 the supposed defects of the photo- 

 graphic method. These defects may priucipally be comprised under the head of 

 Possibilitv of Distortion ; and the importance of an investigation into this source 

 of error will appear at once obvious iu all cases where the position of a definite point 

 with reference to a system of coordinates has to be determined from measured 

 photographs, e.specially in such a retined application of it as that which it will 

 have in the determination of the solar parallax. 



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

 iutriiisic — that is, either optical or mechanical. The instrumental apparatus for 

 producing the image may produce optical irregularities before it reaches the sensitive 

 plate ; or an image optically correct maj^, by irregular contraction of the sensitive 

 tilm in the process of drying, and other incidents of the process, present on the 

 plate a faulty delineation *. 



In general, two waj's present themselves for clearing observations from errors. 

 Either methods may be devised for determining the numerical amount of every error 

 from any source, or by special contrivances the source of error may be contracted to 

 such insif^nificant limits that its effect in a special case is too minute to exert any 

 influence upon the result. Both these roads have been followed in the inquiry into 

 the optical distortion of photogi'aphic images. 



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

 primarv 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 un- 

 alterable and known length, be photogiaphed iu preciselj' the same manner in 

 which celestial events are photographically recorded, the object being at a con- 

 siderable distance ; it may successively be brought into all possible positions in the 

 field of the photoheliograph, and the length of the image on the photograph may 

 be measured afterwards at leisiu-e by means of a micrometer. These lengths will 

 chano-e relatively wherever distortion takes place; but by laying down these 

 varyino- lenoths we shall obtain an optical distortion-map of the particular instru- 

 ment ; and tables may be constructed giving in absolute numbers the con-ectious to 

 be applied to measurements of positions on account of the inffuence of optical 

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

 secondary magnifier is ascertained. The chief source of distortion, if such exist, 

 will be in the secondary magnifier ; and in order to ascertain its amount a reticule 

 of lines drawn at equal distances upon glass may (as has been done recently by 

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

 secondary magnifier. The required data are then immediately given by the 

 measurement of the resulting pictures of the parallelograms on the reticule. Mr. 

 Dallmeyer has ascertained in this manner that no sensible distortion exists in the 

 secondary magnifier constructed by him. The truth of the principle being granted, 

 it was applied to a preliminary series 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. 



* 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 practised, and, moreover, is so much more troublesome, that it does not 

 Beam to be advisable to adopt it, especially as tlie subsequent measurements would present 

 greater difficulties than occur with collodion pictures. 



