CORRELATlON OF CAMERA AND VIEWER 63been discussed we shall confine ourselves largely at this time to
the effects of abnormalities of perspective and parallax without
giving undue regard to balancing such abnormalities.
One important phase of this discussion is that of pointing out
the serious errors which arise, for example when a club or society
holds an exhibition or sends out a postal portfolio composed of
slides made to conform to a variety of standards. This inevitably
means that many stereograms will be examined in stereoscopes
wholly unsuited to the view. As a result, the stereograms are sub-
jected to criticism which is manifestly unfair. These criticisms
should have been leveled at the back of stereo technique displayed
by the critic who failed to observe one of the great fundamental
laws of stereoscopy. 35mm viewers have different focal lengths
but the differences are slight, hence the modern amateurs can ex-
change slides freely. This is far different from the time when
the camera might have had three-inch lenses and the stereogram
be viewed with eight-inch ones.
Variations of Focal Length.-Figure 4-1 shows the visual rays
Fig. 4-1. Fig. 4-2.