2-i2 W. M. C'Am^he\\—^^otes on the Transit of Venus. [Dec. 



tlie G. T. Survey and J. Wafcerliouse, Assistant Surveyor General, with 

 Sergt. J. Harrold, E. E., and two men of H. M.'s 55th Regt. as assistant 

 photographers. 



The instrumental equipment of the party consists of — ■ 



1st. — A very fine 6-in. Equatorial by Cooke and Sons. 



2;^ J.— A new 36-inch Theodolite by Troughton and Simms, lately sent 

 out for the G. T. Survey and lent for the occasion. 



Zrd. — A Photo-heliograph by DaUmeyer. 



4it]i. — A quadruple Chronograph. 



^tli. — An Astronomical Clock which marks the time in seconds on the 

 chronograph and, also by electricity, shows time on three dials, one of which 

 is placed close beside each of the three principal instruments. 



For the shelter and accommodation of the instruments a temporary 

 Observatory has been built comprising a room for the standard clock, chro- 

 nograph and transit instrument, with two attached revolving domes for 

 the theodolite and photo-heliograph. In immediate connection with the 

 latter of these domes is a convenient dark-room for the preparation of the 

 photographic plates. The equatorial is in a separate detached revolving dome. 



The equatorial will be used by Colonel Tennant, who will observe the 

 contacts, take measurements between the cusps of Venus, while she overlaps 

 the sun's limb, with a double-image micrometer, and also probably micro- 

 metrical measurements of her position throughout the Transit. 



He may also use the transit instrument to determine time by daylight 

 transits of bright stars during the phenomenon. 



Captain Campbell will use the 36-in. theodolite to observe the contacts, 

 and throughout the Transit he will take observations of the planet's posi- 

 tion on the sun's disc, by means of a succession of transits of sun and 

 planet, recorded with the chronograph. 



Captain Waterhouse will take photographs on six-inch plates, with a 

 solar image of about 4 inches in diameter, in the photo-heliograph at inter- 

 v^als of two minutes nearly throughout the Transit. He will also, by means 

 of an apparatus constructed by Mr. Warren de la Rue on the principle first 

 proposed by M. Janssen, the eminent French astronomer, and known as 

 the " Janssen slide," endeavour to obtain a series of pictures of the sun's 

 limb and planet together for some seconds, before and after the moment of 

 each of the last three contacts, and also about the times of bisection. It may 

 be explained that the Janssen slide is an ingenious arrangement for carrying 

 a revolving circular sensitive plate about 11" in diameter in such a manner 

 that small radial segments near the circumference may be successively 

 exposed to light at intervals of about one second, thus enabling 60 small 

 pictures of any specific part of the sun's image to be taken in the course of 

 a minute, forming an annular belt about 1 1- inch wide round the circumfer- 



