3. A marker will hold an aiming-disc flat on the paper, the front facing the instructor. The instructor will align the sights on it correctly, and its position on the paper will then be marked by a dot made with a pencil passed through the hole in its centre.
4. Each man in succession will then be called up and ordered to look along the sights, but without touching the rifle, and when he has satisfied himself as to the correctness of the aim, the disc will be removed. It will then be replaced on the paper as before, and moved at his direction until the lower edge of the bull’s-eye is brought into the line of sight, when its position will be marked as before. This operation will be repeated three times in the case of each man, and the points thus fixed will be joined in such a way as to form a triangle.
B. Horizontal
A. Vertical
Fig. 14.—Triangle of Error
5. The position of these points in relation to the instructor’s aim will expose any constant error in aiming. If the recruit’s aim is below that of the instructor, it shows that he has taken too full a sight; if above, that his sighting has been too fine; if to the right, that the foresight was on the left of a perpendicular drawn through the centre of the notch; if to the left, vice versa.
6. The position of the points in regard to one another will show inconsistency. If the triangle is formed so that its greatest side lies vertically on the paper, it proves that the soldier’s principal fault is inconsistency in respect of the amount of foresight taken up into the line of sight ([Fig. 14], A); if the greatest side of the triangle lies horizontally it shows that his principal error is inaccurate centring of the foresight ([Fig. 14], B).
7. If any one of the sides of the triangle exceeds ⅓ inch from the instructor’s aim, the recruit will be noted for further instruction.
8. Aim-Corrector.—The aim-corrector will also be used to enable the instructor to supervise the soldier’s methods and test his progress, the aim being taken from an aiming or firing rest.