LOCAL PARTICULARS OF THE TRANSIT OF VENUS, IS74<. 85 



as it is with eye observations, that favourable conditions should exist for 

 retarding the period of contact at one station and accelerating it at another, 

 because in the photographic method the length of the cords need not be 

 directly considered in determining the nearest approach to the sun and 

 planet's centres. During the transit, photographs at intervals of two or three 

 minutes may be obtained, and any or all of them usefid for comparison with 

 other stations. The epoch of photographic record is determinable with the 

 iitmost accuracy, because the time is not more than l-50th or 1-lOOth of a 

 second, and the instantaneous slide makes an audible signal which niay be 

 recorded or made to I'ecord itself on the chronograph, the small fraction of a 

 second betweeii actual exposure and the striking of the slide may be 

 determined and allowed for.* Figure H represents the solar disc of the 

 dimensions of the photograph which would be taken with tlie Kew photo- 

 heliogi'aph at the transit of 1874, and under tlie same conditions of adjust- 

 ment as existed during the eclipse observations of 1860. On this occasion 

 1-thousandth of an inch represented 0'496 second of arc. Figure 11 is direct as 

 given in that instrument, a b two positions of Venus ; C, the centre of the 

 sun. The dark line marked ingress egress the true path of Venus, and the 

 dotted lines d and e the possible amount of displacement from parallax. 

 The points N. E. S. and W. are the apparent north, east, south, and west 

 points of the sun. 



" In the transit of 1874 the solar disc photographed as above would 

 have a semi-diameter of 1965'8 thousandths of an inch (or a diameter of nearly 

 4 inches), and Venus a semi-diameter of 63'33 of these units, and the pai-allax 

 of Venus refei-red to the sun would be represented by 47'85 of these units ; 

 the maximum possible displacement being 9o'7 units, or nearly 1-tenth of an 

 inch . 



"Means have been devised for ascertaining the distortion of the photograph 

 picture, by which the measures are made satisfactory. By photograpliing 

 a scale of equal parts placed at one or two miles from tlie instrument, a picture 

 will be obtained in which different equal portions of the scale would occupy 

 different places and spaces in the photograph ; and by comparing these by 

 means of a micrometer, the effect of optical distortion can be obtained to 

 a great nicety. Such a scale of equal parts, if erected at a distance of two 

 miles, would have to be about 110 feet long, in order that its image might be 

 somewhat longer than the diameter of a sun picture. It could be made by 

 fixing two horizontal rails one over the other, so that their outer edges would 

 be distant 3 feet. On these, plates of zinc, 2 feet wide and 3 feet long, would 

 have to be fastened so as to leave an interval of exactly 2 feet between adjacent 

 plates. The plates might be made of the same width by clamping the whole 

 series together and planing their edges in a planing machine. In placing the 

 plates on the rails, one might be used temporarily as a template to govern the 

 interval between a plate previously fixed and one about to be screwed down. 

 Of course care must be taken to make the supporting rails firm and straight, 

 and if possible with a sky background. The focus of the Kew heliograph 

 for an object two miles distant would be about l-50th of an inch longer than 

 that for parallel rays. This produces a little indistinctness of outline in the 

 picture, but does not prevent measurements being made. Before finally fixing 

 the enlarging lens for sun pictures, sharply defined pictures of the scnle should 

 be obtained by adjusting the focus. Fears have been expressed that the col- 

 lodion in drying becomes distorted. Experiments, however, in 1860-1861, 

 have demonstrated that the shrinkage is only in the direction of the thick- 

 ness." 



* The electric signal may he given at the exact moment of exposure, hy placing the 

 contact pieces so that they pass as the opening in the slide passes the centre of the opening 

 through which the image passes. — (Note by H. C. R.) 



