Dr. Oliver Wendell Holmes, of America, in 1861, invented a form somewhat like that illustrated at G, but without the sliding view holder, the latter improvement being added by Joseph L. Bates, of Boston, in 1864. The design shown at G has become universal. There are some persons, however, who experience considerable difficulty in seeing stereoscopic views properly with this class of instrument, the reason generally being that the pupillary centres of their eyes are not close enough together or far enough apart to suit the index of refrac tion and magnification of the lenses of the particular stereoscope in use. To obviate this difficulty, stereoscopes have been designed with means for varying the separation of the lenses, Baird's Lothian stereoscope being a good example of this type of instrument ; the proper distance for the pictures in relation to the lenses is arrived at by sliding the view-holder along parallel carrying tubes, the latter being detachable so that the instrument may be used for the examination of stereoscopic prints mounted in albums.
Theodore Brown's " Blocket " instrument (see E) is probably the smallest stereoscope. It is based on the laws of reflection, and acts as follows :—Let the pictures be repre sented as situated at A B, the eyes of the observer at 1, and R, and the optical part at E. The left eye r, sees its picture by direct vision from 1, to A. Inside the small casing E two mirrors are fixed facing each other, but not quite parallel. The right eye R sees in the mirror c an image of the picture B by reflection from mirror ID, and, by a suitable adjustment in the relative angles of these two mirrors, B is superimposed upon A, where stereoscopic fusion takes place. If a prism of the shape of the space between the mirrors C and D is sub stituted for the mirrors and the casing is dis pensed with, a prism stereoscope (shown at F) is the result, and the light will be acted upon precisely in the same manner and with the same result.