1S75.] the Observation of the Transit of Venus. 71 



time to go into the subject very thoroughly nor the means of trying other 



tests. 



Arrangements of the Observatory. — I arrived at Roorkee on the 



13th October and thus had about eight weeks for preparation. Colonel 

 Tennant had built an observatory with domes for all the observing in- 

 struments and had allotted to me a very convenient dark room about 

 ten feet square, attached to the dome in which the photoheliograph had 

 been erected and separated from it by a narrow passage about 7 feet 

 long and 3 feet wide. I had doors placed at each end of this passage, so 

 that communication could pass between the dome and the dark room 

 without letting light into the latter ; and in order to avoid the necessit} 1 - 

 of constantly opening the doors for the passage of the dark slides to and 

 fro, a sort of box opening at both ends and large enough to hold a dark 

 slide was let into the panelling of each of the doors, and the dark 

 slides were thus easily passed backwards and forwards without any risk 

 of letting in light or raising of dust. Double doors were also constructed at 

 the entrance to provide for communication from outside without interruption 

 of the work going on within. Tables and shelves were arranged in the 

 dark room so as to keep all the operations and the necessary chemicals and 

 appliances for each quite distinct ; thus there was a table for the nitrate 

 baths and near it, shelves for the collodions and plate boxes. Another 

 table with sink, was set apart for developing and close by, were shelves for 

 the developers and chemicals &c. used for developing. A third table was 

 used for changing dry plates and above it were shelves for the dry 

 plate boxes. Some such system was absolutely necessary in such import- 

 ant operations, and the principle of a place for every thing and every thing 

 in its place was rigidly adhered to. 



As it was undesirable to use the dark room in the observatory for the 

 preparation of plates and chemicals or as a store room, nothing was kept in it 

 except the chemicals and apparatus actually required there. A dark room for 

 the preparation of dry plates, testing baths, &c, was fitted up in a house 

 immediately opposite the observatory, and here also all mixing of chemicals, 

 cleaning plates, and other preparatory work was carried on and spare stores 

 kept. 



The photoheliograph had been erected by Colonel Tennant before my 

 arrival on an isolated brick pillar in the centre of a circular room 12 feet in 

 diameter, fitted with a revolving observatory dome. 



It was arranged that the times at which the several photographs were 

 exposed should be recorded by electricity on a chronograph placed in an ad- 

 joining room in electric communication with the standard clock, which also 

 gave the time to a clock-dial placed in the dome. 

 10 



