The value of the constants P and Q can be obtained by making deflexion experiments at three distances.
A ray of light from a lamp is thrown on the mirror, whence it is reflected upon a white surface or scale set at a distance of about 3 ft., forming a bright spot on the surface; the slightest angular deflexion of the mirror, owing to its distance from the scale, moves the spot of light a very appreciable distance to the right or left according to the direction of the angular movement.
Its deflexion was observed by an attached mirror in the usual way.
At first a considerable current is indicated by the galvanometer; the deflexion soon diminishes, however, and finally becomes very small.
Taking disease to be a deflexion from the line of health, the first requisite of medicine is an extensive and intimate acquaintance with the norm of the body.