The sidereal messenger of Galileo Galilei : and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries . e telescope, andthe larger with the other eye unassisted; for thatmay be done without inconvenience at one and thesame instant with both eyes open. Then both figureswill appear of the same size, if the instrument magni-fies objects in the desired proportion. After such an instrument has been prepared, the THE SIDEREAL MESSENGER. 13 method of measuring distances remains for inquiry, Method ofand this we shall accomplish by the follow
The sidereal messenger of Galileo Galilei : and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries . e telescope, andthe larger with the other eye unassisted; for thatmay be done without inconvenience at one and thesame instant with both eyes open. Then both figureswill appear of the same size, if the instrument magni-fies objects in the desired proportion. After such an instrument has been prepared, the THE SIDEREAL MESSENGER. 13 method of measuring distances remains for inquiry, Method ofand this we shall accomplish by the following con- LS an^itrivance :— distances between heavenlj bodies by the size of then . ^ aperture of Q the telescope. For the sake of being more easily understood, Iwill suppose a tube A B c d/ Let E be the eye of theobserver; then, when there are no lenses in thetube rays from the eye to the object f g would bedrawn in the straight lines E c F, E d g, but whenthe lenses have been inserted, let the rays goin the bent lines E c H, E D i,—for they are con-tracted, and those which originally, when unaffectedby the lenses, were directed to the object f g, will. The line c h in Galileos figure represents the small pencil of raysfrom H which, after refraction through the telescope, reach the eye enlarged figure shows that if op be the radius of the apertureemployed, the point H of the object would be just outside the field ofview. The method, however, is at best only a very rough one, as theboundary of the field of view in this telescope is unavoidably indistinct. 14 THE SIDEREAL MESSENGER. include only the part h i. Hence the ratio of thedistance e h to the line h i being known, we shall beable to find, by means of a table of sines, the magni-tude of the angle subtended at the eye by the objectH I, which we shall find to contain only some if we fit on the lens CD thin plates of metal,pierced, some with larger, others with smaller aper-tures, by putting on over the
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Keywords: ., bookauthorgalileigalileo1, bookcentury1800, booksubjectastronomy
