THE STEREOSCOPE 47Fig. 3-10. Courtesy Keystone View Co.Another type is the split slide which is actually two separate
slides, each having a portion of a picture upon it. Thus there may
be a star upon one slide and a circle upon the other. In this in-
stance the star is supposed to be seen within the circle, and to
achieve this result the two cards are moved farther apart or nearer
together in a calibrated holder.
In still another type, a special desk form of stereoscope is used
and the cards have a simple outline picture such as a flower, vase,
boat, or the like, printed upon one end only. The subject places
this in the stereoscope and then proceeds to draw the second image
upon the blank end of the card. When this can be done success-
fully the eyes are exceptionally well coordinated. The use of this
stereoscope develops both coordination and stereopsis to a marked
degree.
Although these slides are known generally as therapeutic ones,
they are of interest and of value to every stereographer. Unfortu-
nately space does not permit a detailed account of the experiments
and exercises which anyone may use at home, but the information
is readily available to those interested.
Similar therapeutic slides are made which do not use the stereo-
scope but which make use of Vectograph slides, such as the Bausch
& Lomb Orthofusor.
CHARACTERISTICS OF THE STEREOCRAM.-AS we shall see in the
next chapter, the use of the stereoscope is essential for repro-
ducing the orthostereoscopic (true stereo) effect, and to produce
this effect the PePax principle is fundamental. This principle