An engineer is calibrating a horizontal spring system designed to launch a small projectile. The spring is fixed at one end and stretched horizontally along a flat bench next to a parallel millimetre scale. To determine the spring constant, the engineer records the position of the free end of the spring under different tension forces.
The engineer finds it difficult to obtain highly accurate and consistent readings of the spring's extension because of potential parallax error, as shown in the diagram.
Describe one modification the engineer could make to the apparatus or procedure to improve the accuracy of their measurements.