LOCAL PARTICULARS OF THE TRAJ!TSIT OF VENUS, 1874. S7 



"Tte quantity and direction of the inevitable distortion is ascertained in 

 two ways: — 1st, By placing in the focus of the objective a plate of plane 

 parallel glass upon which a reticule of lines is engraved, and taking a photo- 

 graph of this when the focus is exactly adjusted. If then the reticule 

 and the photo are compared, the amount of distortion produced by the 

 enlarging lens will be known. 2. Place in the position of the sensitive plate 

 a plate of parallel glass ; upon which lines running north and south (transit 

 lines) are engraved about three seconds of equatorial time apart. Stop down 

 the object glass to about one inch, and clamp the telescope firmly. Then by 

 means of the chronograph take transits of many bright stars over all the lines ; 

 observing them with the eyepiece before mentioned. A comparison of these 

 transits will give not only the angular value of the intervals between the lines, 

 but also the amount of distortion produced by the instrument, objective and 

 enlarging lens included." 



" A second method of ascertaining the angular value of the parts of the 

 field is to photograph a known object at a known distance, such as a building 

 by day, or two electric lights expressly arranged at night ; or a group of stars 

 like the Pleiades, whose distances are accurately known. By a combination of 

 these methods I think the angular value of a given linear space may be 

 accurately known. But with all these precautions it is important that no 

 reliance should be placed, for precision, upon the apparent outline of the sun 

 at any particular part ; for the photograph of the sun will have a greater or 

 less diameter, by many seconds, according to the energy of the sun's rays, by 

 a change in apertiire of the telescope, by the time of exposure, or the hour 

 of the day at which the picture is taken, or by the sensibility of the chemicals. 

 You may be tempted to say, if this be true, what reliance can be placed upon 

 the results of photography ? I answer, that the sun has no defined outlines 

 at any time, even to the eye ; but in its best state is an irregular, seething, 

 ever restless object, utterly unfit to be the starting point for measures of 

 precision, and that while the eye is confined in its attempts at measures to 

 some small part of the sun's limb, the photograph can be placed upon the 

 stage of the micrometer, and accurately centred with reference to the average 

 of the whole contoui", and thus escape the errors sure to be the result of 

 measures based upon local bisections." 



" I would recommend for photographing the transit an achromatic object 

 glass, 5-inch aperture, 70 inches focus, reduced to 60 inches by the front lens 

 before described, and an enlarging combination which makes the sun's image 

 2 inches diameter at 70 inches from the objective. I would make the camera 

 box part of the tube, and the focal adjustment near the object glass." 



Professor Newcomb says : — 



" It is probable 200 negatives may be taken at each station, and it is to be 

 hoped that the average error of the whole will not exceed 0'03. It is desirable 

 that the constant error peculiar to the station should not exceed a fraction 

 of this, say 0'02 ; this would require that all the arrangements should be so 

 made that the error in measuring the distances between the centres, which on 

 the average is about 800 seconds, should not exceed l-40000th of the entire 

 space measured (i.e., one-fiftieth of a second) ; and in order to mate photography 

 useful as compared with other methods, it is necessary that the" error should 

 not exceed 1-lOOOOth part of the space measured (if the photos are taken 

 4 inches diameter this becomes l-7000th part of an inch). To secure corre- 

 sponding accuracy in measuring the angle of position, it is necessary that the 

 zero line should be correct within 15", or, in other words, on the same scaled 

 photo, the position of the end of the zero line must be known within about 

 l-6000th of an inch." 



