THE STEREOSCOPE : ITS HISTORY. 



51 



is that of magnifying the pictures as they are binocularly viewed. It 

 was indeed a happy thought that produced such a combination of ad- 

 mirable 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 e 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 mag- 

 nify 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 dis- 

 carded 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 Ameri- 

 can whose name seems to be but little associated in the popular mind 



Fig. 11. Accommodating Grandchildren, 1882. 



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 profess- 

 or,' 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 thou- 

 sands, until now no home is too humble, no father too poor, to delight 



