include only the part h i. Hence the ratio of the distance e h to the line h i being known, we shall be able to find, by means of a table of sines, the magnitude of the angle subtended at the eye by the object h i, which we shall find to contain only some minutes. But if we fit on the lens c d thin plates of metal, pierced, some with larger, others with smaller apertures, by putting on over the lens sometimes one plate, sometimes another, as may be necessary, we shall construct at our pleasure different subtending angles of more or fewer minutes, by the help of which we shall be able to measure conveniently the intervals between stars separated by an angular distance of some minutes, within an error of one or two minutes. But let it suffice for the present to have thus slightly touched, and as it were just put our lips to these matters, for on some other opportunity I will publish the theory of this instrument in completeness.
Now let me review, the observations made by me during the two months just past, again inviting the attention of all who are eager for true philosophy to the beginnings which led to the sight of most important phenomena.
The Moon. Ruggedness of its surface.Let me speak first of the surface of the Moon, which is turned towards us. For the sake of being