The pin looks exactly on the image from where you are. Move your head to one side, then the other, and watch.
01Put the object pin where the question says, and note its distance on the rule.It’s the fixed thing in the experiment. Everything you find is measured relative to the mirror, and the object distance is what your answer will be compared against.
02Stand the search pin behind the mirror, roughly where you think the image is.Roughly is enough to start. The method is a test-and-adjust, not an act of aim, and the pin must be tall enough to be seen over the mirror while the image is in the glass below.
03Look from one side of the normal and note which way the pin sits against the image.This is the measurement. From the normal there’s nothing to see; the information is entirely in the sideways views.
04Move to the other side and look again.If the pin swaps sides, you have bracketed the image between the two apparent positions, the same logic as bracketing a null, on a different bench.
05Move the pin, and repeat, until it stays with the image from both sides.Adjusting after looking from only one side will chase the setting back and forth. Look both ways, then decide which way to move and by how much.
06Lean in and check once more before you write anything down.A separation invisible at arm’s length is plain when you look closely. The last check is the one that turns a plausible setting into a measurement.
07Read the pin’s position off the rule, to the nearest millimetre.The rule is what the answer comes off. The pins and the mirror are the apparatus that puts the pin in the right place; they’re not what you record.
Laboratory
You’re looking straight down the normal. From here the pin, the image and the object all lie on one line, so they appear together however badly the pin is placed. This is the one viewing position that can’t tell you anything. Move your head to one side, then the other.