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238 changes to be made, and if every change took ten seconds we should be running it rather fine. Fortunately the changes can be made more quickly than this. Special plate-holders are arranged which open like a hinged door instead of like a sliding door as usual; and to ensure rapidity the astronomer has two assistants, one to hand him each plateholder when he wants it, the other to take it from him when exposed. In this way he can change plates in six or seven seconds. There are nevertheless a good many things to do in this time. After giving word to the man at the lens to put on the cap (which for eclipse work may be a light Japanese fan held in front of the lens without touching it) he must close the door of the slide, take it out of position and hand it to one assistant; receive the new slide from the other assistant, put it in place in the camera, open the door and (after, perhaps, allowing a second for the instrument to settle) give the word for the cap to be removed. With a little practice this can be done in six seconds, and these gentlemen have very kindly been practising this morning so that they may not fail to carry through the programme smartly this afternoon. One of them I have not yet mentioned the time-caller. It is his business to look at a watch and call out seconds in a loud voice from the moment the eclipse commences, so that the astronomer may make the exposures of the right length, and know how time is passing. Thus suppose that when the astronomer has put in the holder for the fourth exposure of ten seconds, the time-caller is calling seventy-nine; the astronomer will know that he