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38 Ophthalmic Lenses

(clockwise), then to the left (anti-clockwise). If a section of the door frame
appears slanted, it establishes that the lens under examination is a
cylindrical lens. This is called “scissors like movement”.

Fig. 4.11: Looking through a plano-cylinder, a section of the
door’s frame appears to be slanted

Fig. 4.12: Upon rotating the lens, a position will be found
in which the door frame appears straight

Next step is to find out two principal meridians of the lens. To do so
continue rotating the lens and at one position on rotation, the frame of the
door would not look slanted. This is one of the principle meridians. The
meridian opposite to this is the other principal meridian. They are at 90
degree to each other. Now, neutralize both the meridians separately. If the
movement in one meridian is nil, it is a plano-cylindrical lens and nil
movement meridian is taken as the axis of the lens. Contrarily, if both the
meridians show movement, it is a sphero-cylindrical lens. The weaker
movement meridian is taken as the axis of the lens under observation. Now,
neutralize both the meridians as before to determine exact spherical and
cylindrical elements.