(14) TRANSIT OF VENUS, 1874. APPENDIX. 



APPENDIX V. PHOTOGRAPHIC OBSERVATIONS of the TRANSIT of VENUS. 



The apparent uncertainty in the conclusions from the photographic registers has led extensively 

 to the persuasion that it is unnecessary to record the photographic operations with the utmost 

 detail. But it appears proper to explain the views which directed the order of reductions, and to 

 describe the instruments which have been employed. 



The photoheliograph instruments in every case were mounted equatoreally, and it is probable 

 that their adjustments were, as for ordinary uses, reasonably accurate. But it is impossible to place 

 so much confidence in these adjustments, or in the insertion of the photograph-plate in the tube, 

 as to justify the supposition that differences of R. A. and of N. P. D. between the Sun and the 

 Planet could be measured with even moderate accuracy. The only measure upon which reliance 

 could be placed is that of the distance between the centers of the two objects. Relying on the 

 practicability of putting the plates fairly in position for that measure (extreme accuracy in this 

 adjustment is not indispensable) it was only necessary to provide means of measuring accurately 

 in one direction. 



The principle of the micrometer here adopted is the following : Two microscopes, both pointing 

 downwards, pass through a very firm bar, 15 inches long ; the tubes of the microscopes being 

 separated about 11^ inches. (The bar, in fact, is not a solid piece of metal, but is formed like a 

 box, with sides 2| inches deep, and is free f'rotn sensible flexure under any ordinary strain.) The 

 two microscopes, in this state, form a microscopic beam-compass. The beam-compass can be slid 

 endways, with a range of nearly 8 inches, in an intermediate frame about 23 inches long, to which 

 it can be clamped at any part of its motion ; practically, however, the clamp was not used. 

 That intermediate frame has a slot or chase extending nearly from end to end, through which the 

 microscopes, and the bar which carries them, pass. The intermediate frame itself can be moved 

 endways (carrying with it the beam-compass) upon the upper face of the fixed stand, by means 

 of a fine screw at one end, carried by the fixed stand ; the range of this screw is less than 1 inch. 

 The upper part of the fixed stand is a strong plate, 24 inches long, slotted nearly from end to end, 

 in the same manner as the intermediate frame ; and the 'beam-compass slides endways through 

 the slot of the intermediate frame and through the slot of the upper part of the fixed frame at the 

 same time. The upper part of the fixed frame is earned by pillars 3 inches long fixed to tke 

 base-plate. The base-plate is 24 inches long and 7 inches broad, and it has two longitudinal 

 perforations each about 9 inches long and 3 inches broad, with an intervening metallic cross-bar 

 2 inches in breadth. One of the microscopes always points downwards to one of these perforations, 

 and the other microscope to the other perforation. The wooden structure which carries the 

 base-plate has corresponding perforations. In use, the wooden structure is so planted above a 

 mirror that the light of the sky is reflected upwards through the microscopes. The microscopes 

 are now in a fit state for observing transparent objects which are laid upon the base-plate. 



Over one of the perforations of the base-plate is laid the photograph, of which measures are to 

 be taken. On the method of placing this photograph we shall speak shortly. The microscope 

 which views the photograph, and which we shall call the " photograph-microscope," lias a crossed 

 wire, fixed in the field of view. 



On the other perforation of the base-plate there is placed, in a, receptacle adapted to its form, 

 the " millimetre scale ; " a scale etched on glass, graduated to 240 millimetres ; its length is parallel 

 to the length of the beam-compass. The microscope which views the scale, and which we shall 

 rail the " scale-microscope," has the usual cross-wires carried by a micrometer-screw. 



