Page 134 - July-August 2020

shows an effect known as prism of the line, the path lengths of the (Figures 3(d), (e) and (f) and finally to
metamorphopsia, and it is seen that rays through the prism increase in a combination of these two angles
horizontal lines appear to curve length, thereby subtending a larger (Figure 2(g)).
upwards when viewed through angle at the apex of the prism, with 1
sections of the prism held with its the result that the effective apical Ogle pointed out that there are
base downwards, in which the angle, and hence the deviation five basic types of prism distortion,
path lengths through the prism produced by the prism, is increased. namely curvature of lines, vertical
gradually increase. and horizontal magnification,
Figure 4 illustrates the appearance asymmetric versions of either
The curvature can be understood of the target of Figure 3(a) when of these last two and oblique
from the section taken from the changes are made, first to the asymmetric magnification.
isometric view of the prism where pantoscopic angle of the prism,
it is seen that, as the eye views the (Figures 3(a), (b) and (c)) and then Vertical magnification is seen
horizontal line away from the centre to the face form angle of the prism when the pantoscopic angle of the
prism is increased, in the sense that
the apex of the prism is positioned
further from the eye as shown in
Figure 4(a). If a negative pantoscopic
angle is applied, the apex of the
prism being moved towards the
eye, the squashed appearance
(fractional magnification) shown in
Figure 4(b) is seen. As the tilt of the
prism is increased it is seen that the
effect increases towards the prism
apex giving rise to the asymmetric
appearance shown in Figure 4(c).
Figures 4(d), (e) and (f)
show changes to the horizontal
magnification as the face form angle
is altered when a prism base out is
tilted about a vertical axis before the
eyes. When the face form angle is
increased, the appearance shown in
Figure 4(d) is observed. If a negative
face form angle is applied, the apex
of the prism being brought nearer
to the face, the appearance shown
in Figure 4(e) is obtained. Increasing
the face form angle of the prism
reveals the asymmetric nature of the
magnification (Figure 4(f)).

If the plano prism is tilted about
FIGURE 4. MAGNIFICATION EFFECTS WHEN TARGET SHOWN IN FIGURE 3(A) IS VIEWED both axes at the same time the
THROUGH PRISMS WITH PANTOSCOPIC AND FACE FORM ANGLES square object takes on the form of
4(A) - (C) EFFECTS DUE TO PANTOSCOPIC ANGLE a parallelogram the limbs tilting
4(D) - (F) EFFECTS DUE TO FACE FORM ANGLES
4(G) EFFECTS DUE TO A COMBINATION OF PANTOSCOPIC AND FACE FORM ANGLES noticeably as shown in Figure 4(g).

| JULY-AUG 2020 | 130 LENS TALK