Page:A Voyage in Space (1913).djvu/257

Rh eclipse only lasted one second before (say) clouds came up, the astronomer would have got something to take away. Hence, he puts that short exposure first. Next he ventures on five seconds; and then when he has got those two he ventures on a long exposure of forty seconds, so as to photograph the faint parts of the corona.

A ten-second and two-second exposure complete the programme of five. Altogether, therefore, the camera will be open for 1 + 5 + 40 + 10 + 2 = 58 seconds, and since the whole eclipse is to last 100 seconds it looks as though we might take more time than this; but you must remember that it requires some seconds to change the plate for the next photograph. There are four