1S75.] the Observation of the Transit of Venus. 65 



The superintendence of the official arrangements for the observation 

 of the Transit in Northern India was entrusted to Colonel Tennant, 

 R. E., who has done so much to further the progress of astronomy and 

 solar physics in this country, and was one of the first to recognise the 

 value of photography as a means of recording the Transit. He selected 

 Roorkee in the N". W. Provinces as his station of observation, partly on 

 account of the great advantages to be gained by the proximity of the Canal 

 Workshops for setting up the observatory and the repair and adjustment of 

 instruments. 



Photoheliograph. — Itwas arranged that photographic observations should 

 form part of Colonel Tennant's programme and that with this object he should 

 be furnished with a photoheliograph by Dallmeyer, of the same construction 

 as those supplied to the English and Russian expeditions. These instruments 

 were on the same principle as the photoheliograph designed by Dr. "Warren 

 De la Rue for the Kew Observatory, and consisted of a telescope com- 

 bined with a photographic camera, ecpuatorially mounted, and driven by 

 clockwork. According to a description given by the maker, the object 

 glass was 4- in. diameter and 60 in. focal length, corrected to com- 

 bine the chemical and visual foci. The image of the sun formed at the prin- 

 cipal focus was about ^ in. in diameter and was thrown on to an enlarging 

 combination by which an enlarged image about 4 in. diameter was projected 

 on to the sensitive photographic plate arranged as in an ordinary camera. A 

 little in front of the enlarging lens was a slide pierced with two circular 

 openings, one fitted with spider-web crosslines and the other with a glass 

 plate ruled with a fine reticule of squares, and capable of adjustment so as 

 to be brought into the focus of the object-glass in order that the cross-wires 

 and reticule might be enlarged and brought to fine focus at the same time 

 as the image of the sun. The pictures could thus be taken with the cross- 

 wires, which served as a reference mark for measurements in connection with 

 the declination and right ascension circles, or with the reticule, by means 

 of which any optical distortion caused by the secondary enlargement of 

 the image could be measured. 



The quick exposure of the plates was effected by means of a shutter 

 sliding between the cross-wires and the enlarging lens, in which position the 

 object could be effected with a minimum of motion. This shutter was held 

 at its lower end by a spring and was arranged so that when raised to its 

 full extent, by means of a string attached to its upper end, the passage of the 

 solar rays to the sensitive plate was cut off. This string passed over a 

 pulley on the body of the instrument and had at the end a hook on which 

 a loop of strong cotton thread was attached and, being stretched so as to 

 pass over a conical block fixed on the camera, retained the shutter in its 

 raised position. When the thread was cut, the force of the spring imme- 



