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118 telescope (firmly attached to the photographic one and moved round by the same clockwork), he watches to see that the clockwork is going smoothly and accurately. If it is not, he has the means of correcting it by pressing an electric button. But it is very tiring to watch for long hours like this, and he is generally glad when the first signs of coming daylight warn him to put the cap on his telescope. Sometimes even then he knows that he has not given a long enough exposure; the object may be so faint that even a long winter night of twelve hours is not long enough for it. What is he to do then? Why! he must cover up his telescope most carefully so that no light can get to the plate during the day: and when darkness comes again, he must turn the telescope to the east where the object will rise, set his clockwork going again, take off the cap, and spend another night watching to see that all goes well. Perhaps even two nights are insufficient, so that he must add a third and a fourth: and I need scarcely remind you that all nights are not fine; when there comes a wet or cloudy night, the astronomer must keep his telescope covered up and wait until the weather improves. So that sometimes a plate has been kept in the telescope for many months before the full exposure has been given. You may not have realized what long hours of waiting some of the beautiful pictures of the heavens which you can now see so easily may have cost!

Now we can return to the way of working a big telescope. One of the most recent modern improvements is to attach the clockwork, not to the telescope itself, but to a mirror which reflects the