Lodge “Pioneers of Science.”
Fig. 3.—Galileo.
Laden with honors he returned to Padua and settled down to the hard work of development, grinding many lenses with his own hands and finally producing the instrument magnifying some 32 times, with which he began the notable succession of discoveries that laid the foundation of observational astronomy. This with another of similar dimensions is still preserved at the Galileo Museum in Florence, and is shown in the Frontispiece. The larger instrument is forty-nine inches long and an inch and three quarters aperture, the smaller about thirty-seven inches long and of an inch and five-eighths aperture. The tubes are of paper, the glasses still remain, and these are in fact the first astronomical telescopes.
Galileo made in Padua, and after his return to Florence in the autumn of 1610, many telescopes which found their way over Europe, but quite certainly none of power equalling or exceeding these.
In this connection John Greaves, later Savilian Professor of Astronomy at Oxford, writing from Sienna in 1639, says: “Galileus never made but two good glasses, and those were of old Venice glass.” In these best telescopes, however, the great Florentine had clearly accomplished a most workmanlike feat. He had brought the focus of his eye lens down to that usual in modern opera glasses, and has pushed his power about to the limit for simple lenses thus combined.
The lack of clear and homogeneous glass, the great difficulty of forming true tools, want of suitable commercial abrasives, impossibility of buying sheet metals or tubing (except lead), and default of now familiar methods of centering and testing lenses, made the production of respectably good instruments a task the difficulty of which it is hard now to appreciate.
The services of Galileo to the art were of such profound importance, that his form of instrument may well bear his name, even though his eyes were not the first that had looked through it. Such, too, was the judgment of his contemporaries, and it was by the act of his colleagues in the renowned Acaddemia dei Lincei, through the learned Damiscianus, that the name “Telescope” was devised and has been handed down to us.
A serious fault of the Galilean telescope was its very small field of view when of any considerable power. Galileo’s largest instrument had a field of but 7′15″, less than one quarter the moon’s diameter. The general reason is plain if one follows the rays through the lenses as in Fig. 4 where AB is the distant object, o the objective, e the eye lens, ab the real image in the absence of e, and a′b′ the virtual magnified image due to e.
It will be at once seen that the axes of the pencils of rays from all parts of the object, as shown by the heavy lines, act as if they diverged from the optical center of the objective, but diverging still more by refraction through the concave eye lens e, fall mostly outside the pupil of the observer’s eye. In fact the field is approximately measured by the angle subtended by the pupil from the center of o.
To the credit of the Galilean form may be set down the convenient erect image, a sharp, if small, field somewhat bettered by a partial compensation of the aberrations of the objective by the concave eye lens, and good illumination. For a distant object the lenses were spaced at the difference of their focal lengths, and the magnifying power was the ratio of these, fo/fe.