Page:Popular Science Monthly Volume 21.djvu/59

Rh is that of magnifying the pictures as they are binocularly viewed. It was indeed a happy thought that produced such a combination of admirable features.

Much space could be occupied in describing the many forms of stereoscope that have been devised since that of Brewster was first put forth. They have all been applications of the principles already explained in connection with the reflecting and refracting instruments, devised in 1838 and 1849. That of Helmholtz is probably the best in Europe. In this each tube extends into the box, and is provided with a pair of accurately centred plano-convex lenses, which greatly magnify the pictures. It is indeed simply a pair of telescope eye-pieces, each of which is screwed into a plate to which lateral motion, for the purpose of adjustment, may be given with a screw, lever, and spring. To avoid the necessity of optic divergence, the stereograph must be comparatively small. Such an instrument is necessarily quite costly. The form most widely employed in Europe is that shown in Fig. 8, in which the box is divided by a partition (s), which does not extend so far as to prevent ready motion of the slide. The tubes are discarded and the semi-lenses are permanently fixed, edge to edge (Fig. 9), into the wood at the smaller end. This is objectionable, because no adjustment is possible for either the distance of the card or the width between the eyes.

Twenty years ago the stereoscope just described was the only one extensively used in America. At present it is hard to find, because totally displaced by another instrument, the device of a modest American whose name seems to be but little associated in the popular mind



with his own invention. This fact would be inexplicable were it not that he has made so many thousands of readers happy by his writings on literary topics that they think of him only as the poet, the professor, the genial "autocrat of the breakfast-table," whose delicate humor and warm human sympathy have so often caused smiles and tears to mingle together, that they forget him as the physiologist, who finds use for other instruments besides his mirth-provoking pen. There are few who think of him as an inventor, when they use the convenient and compact stereoscopes that have been multiplied in tens of thousands, until now no home is too humble, no father too poor, to delight