In an experiment to determine the focal length of a concave mirror by no-parallax method, the concave mirror is placed on the base of a retort stand and a pin, P, is held horizontally above it (with the aid of a clamp and cork) as in Fig. 1 above. The clamp is adjusted until an inverted image of the pin is seen when viewed vertically from above. The clamp is then moved up and down, while watching the image position, until a position is reached where the pin P and its inverted image are in line (Fig.1). The distance, d, between this position and the base ofthe retort stand is measured and recorded. Pin Pis then moved to a distance X=20cm from the base of the retort stand. Another pin O is held horizontally and moved up and down until it is in line with the inverted image of P (Fig.2). The distance, y, of pin Q from the base is measured and recorded. The experiment is repeated for X = 22.0,24.0, 26.0 and 28.0cm, in turns, with the corresponding value of determined and recorded in each case. The table below is an example of readings from such an experiment. 1 x- ) y(cm) X (cm X(cm) ( 20.0 22.0 24.0 26.0 28.0 1