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knows just as much of the subject as I do, and that is nothing at all.

No discovery more surprised the whole world than that of two satellites of Mars by Professor Asaph Hall, at the Naval Observatory, in 1877. They had failed of previous detection owing to their extreme minuteness. It was not considered likely that a satellite could be so small as these were found to be, and so no one had taken the trouble to make a careful search with any great telescope. But, when once discovered, they were found to be by no means difficult objects. Of course the ease with which they can be seen depends on the position of Mars both in its orbit and with respect to the earth. They are never visible except when the planet is near its opposition. At each opposition they may be observed for a period of three, four, or even six months, according to circumstances. At an opposition near perihelion they may be seen with a telescope of less than twelve inches diameter; how small a one will show them depends on the skill of the observer, and the pains he takes to cut off the light of the planet from his eye. Generally a telescope ranging from twelve to eighteen inches in diameter is necessary. The difficulty in seeing them arises entirely from the glare of the planet. Could this be eliminated they could doubtless be seen with much smaller instruments. Owing to the glare, the outer one is much easier to see than the inner one, although the inner one is probably the brighter of the two.