Inaction saps the vigor of the mind.

The variety of colour in objects cannot be discerned at a great distance, excepting in those parts which are directly lighted up by the solar rays.

In the days of thy youth seek to obtain that which shall compensate the losses of thy old age.

It is useful to constantly observe, note, and consider.

The eye transmits its own image through the air to all the objects which face it, and also receives them on its own surface, whence the "sensus communis" takes them and considers them.

It is no small benefit on finding oneself in bed in the dark to go over again in the imagination the main lines of the forms previously studied, or other noteworthy things conceived by ingenious speculation.

There is no higher or lower knowledge, but one only, flowing out of experimentation.