“There is always more or less irregularity of motion, all of which would be recorded on the plate, and the stars, instead of showing as merely points of light, would be elongated and blurred. The fainter ones would not show at all, because they could not be still long enough to have their pictures taken. That is why you see in the photograph the observer with his eye ‘glued to the telescope,’ watching a star, a guiding star which he constantly keeps behind the intersection of two illuminated spider threads in the eyepiece, by the slow-motion rods which are controlled by his hands.” Thus, every star or comet is kept immovable on the sensitive plate, and it paints its own portrait as long as the telescope is made to turn westward as fast as the earth rotates eastward. That is why a driving clock is absolutely necessary for the amateur comet photographer who is desirous of obtaining accurate results. Many hours are required in obtaining a successful photograph of such comets as the Morehouse comet, the sensitive plate requiring sometimes an exposure of many hours before it reveals satisfactory results. The observer must sit patiently hour after hour, guiding the instrument, and the writer has some idea of what this must mean, from a brief five minutes’ experience at Mount Wilson in connection with the sixty-inch reflector. As a great favor she was allowed to hold the bulb in her hand which by the slightest pressure brought back an erring star which had attempted to stray momentarily from the center of the field of view of the telescope. When one considers the hours spent by the late Professor Barnard in this nerve-racking work, the patient endurance of the astronomers who specialize in celestial photography becomes evident. It is an arduous task, and one doubtless subject to many disappointments, to avoid which Professor Barnard tried to formulate some set of rules that would be dependent on the local time and position of the comet, but these were finally rejected. “So much would depend on the purity of the atmosphere at the time, the size and light ratio of the lens, the kind of plate used, etc., that they would probably lead to the very errors against which we wish to guide.” The position of the comet with respect to the point of sunrise or sunset, and freedom from any form of haze in the sky, are important factors in the exposure of comet plates. Moreover, it is necessary that they should not be exposed too early in the evening or too late in the morning, in either case resulting in unsatisfactory negatives. The best of all rules is the judgment of the observer at the moment, but only long experience will warn one by a glance at the sky when there is danger of failure in this class of work. It is the few moments at the beginning or end of the exposure that will injure or ruin the plate.