SPECIAL PHASES 301necessary to work only with homologous guide lines, which are
numbered to facilitate identification.
Freehand Drawing.-The best freehand drawings are actually
made freehand. The first is drawn just like any freehand drawing.
The second drawing is then made upon a piece of tracing paper
placed over the first and during the tracing the lines are separated
to right or left according to the spatial orientation of the detail
being shown.
It has been reported that there are a few artists who can make
the first drawing, and then, by using dissociated convergence, they
can draw the second unit, being guided by the actual stereo relief
developed by the drawing itself. If this report is true, such people
must be very few, although there is no logical reason why the
faculty should not be developed.
As far as we have been able to ascertain, no material covering
this phase of stereoscopy has been published other than the Stereo
Guild Manual of Stereoscopic Drawing.
Stereo Therapeutics.-A great many people refuse to engage in
stereo for fear of injuring eyesight. Much of this fear can be traced
to very definite injuries of the kind in the past, because stereo
viewing can be injurious if the stereoscope used is of inferior de-
sign or badly out of alignment due to age or abuse.
Even though it does constitute repetition, we must say again
that stereoscope quality is of vital importance and that there is no
excuse for the existence of cheap viewers which are poorly made.
If you use a 98-cent viewer you are making exactly the same error
you would make if you were to buy your spectacles at the ten-cent
store instead of having them made by a competent optician, The
stereoscope is an accessory to your vision, and it should be of un-
questionable optical quality. This does not mean that it has to be
extremely costly. Good imported viewers still run around $50
and more, but we have domestic 35mm stereoscopes which sell
as low as $10 and which are of satisfactory quality.
Provided the stereoscope used is good, there is no question
about stereo being definitely beneficial. Ophthalmologists pre-
scribe definite stereo exercises which are carried out by means of a
standard type of stereoscope, and which make use of stereograms
of the usual type. The subjects are selected, of course, and the