following Experiments perpendicular to the incident Rays. About this
Axis I turned the Prism slowly, and saw the refracted Light on the Wall,
or coloured Image of the Sun, first to descend, and then to ascend.
Between the Descent and Ascent, when the Image seemed Stationary, I
stopp'd the Prism, and fix'd it in that Posture, that it should be moved
no more. For in that Posture the Refractions of the Light at the two