(18) TRANSIT OF VENUS, l8?4. APPENDIX. 



investigation in the "Observatory," No. 16, Vol. ii., p. 122) the correction found from tiit- 

 observation of the Scale is justly applicable to the photographs of the Transit. It is believed that 

 this mode of adjustment was always employed ; and the corrections adopted in the use of Form 21 

 are made on that principle. If, however, the adjustment were made by altering the place of the 

 photographic plate, a different correction would be required. 



The method of so placing the photograph-plate that the centers of the Sun and Venus would 

 be in the longitudinal line of the measuring apparatus has been described. The crossed wire of 

 the photograph-microscope can then be placed upon the 1st limb of the Sun, the 1st limb of Venus, 

 the 2nd limb of Venus, and the 2nd limb of the Sun. The further treatment of the photographs 

 will be understood from the inclosed specimen of Form 21. The line " Correction for difference 

 of irradiation " was, in fact, never used. 



In some instances, the definition of the limb of Venus was not satisfactory. In particular, in 

 the earlier observations at Mokattam (where the Sun rose with Venus on his disk) some of the 

 images of the planet were much distorted. To obviate this, Major Tupman caused to be prepared 

 a glass diaphragm, on which was engraved a circle somewhat smaller than the disk of Venus, and 

 also crossed lines similar to the usual crossed wires ; and measures were taken for comparing the 

 place of intersection of the wires with the center of the circle. 



The number of sheets of Form 21 employed in calculation is about 440. A portion of the 

 measures was made by Major Tupman ; the greater part, however, by Mr. Burton : no other 

 observer was employed on them. 



The clock-time of every exposure of the photoheliograph- plate for image having been noted, 

 this time was converted into Greenwich Sidereal Time, the tabular distance of centers was com- 

 puted, and the factors of the various possible errors (S n, 8 R.A., 8 N.P.D, 8 1, 8 R, S r) were 

 formed, by the use of Forms 17, 18, 19, 20, exactly as for the eye-observations. The comparison 

 of each of these tabular lines with the distance of centers found in Form 21 gave an equation. 

 These equations were most carefully grouped by Major Tupman ; and solved, by the use of proper 

 factors, in the form most favourable to accuracy. 



After laborious measures and calculations it was thought best to abstain from publishing the 

 results of the photographic measures as comparable with those deduced from telescopic view. 

 The considerations which led to this decision are stated by Major Tupman in a paper in the 

 Monthly Notices of the Royal Astronomical Society, volume 38, page 508 (1878, Supplementary 

 Notice). They are mainly embodied in this : that, however well the Sun's limb on the photograph 

 appeared to the naked eye to be defined, yet on applying to it :i microscope it became indistinct 

 and untraceable, and when the sharp wire of the micrometer was placed on it it entirely 

 disappeared. 



A great number of photographs (216 in all) have been received from the following stations, 

 and are entered in Form 21: From India: Roorkee, 91. From Australia: Sydney, 60; 

 Melbourne, 29 ; Woodford, 20 ; Eden, 16. 



