“Certein bodies… become luminous when heated. Their luminosity disappears after some time, but the capacity of becoming luminous afresh through heat is restored to them by the action of a spark, and also by the action of radium.”

Source: De architectura (The Ten Books On Architecture) (~ 15BC), Book II, Chapter VI, Sec. 4

This being the case, it is evident that the onus probandi [burden of proof] ought to lie with those who are willing to establish such an hypothesis, for it does not appear that Nature is in the habit of using one and the same mechanism with any two of our senses. Witness the vibration of air that makes sound, the effluvia that occasion smells, the particles that produce taste, the resistance or repulsive powers that affect the touch—all these are evidently suited to their respective organs of sense.
Source: Sir William Herschel: His Life and Works (1880), Ch.4 "Life and Works" on his discovery of the infrared.

Source: The Education of the Will (1920), p. 100

The Notebooks of Leonardo da Vinci (1883), IV Perspective of Disappearance

The Notebooks of Leonardo da Vinci (1883), IV Perspective of Disappearance

The Notebooks of Leonardo da Vinci (1883), III Six books on Light and Shade
Context: Shadow is not the absence of light, merely the obstruction of the luminous rays by an opaque body. Shadow is of the nature of darkness. Light is of the nature of a luminous body; one conceals and the other reveals. They are always associated and inseparable from all objects. But shadow is a more powerful agent than light, for it can impede and entirely deprive bodies of their light, while light can never entirely expel shadow from a body, that is from an opaque body.

Pearls of Wisdom

The Notebooks of Leonardo da Vinci (1883), II Linear Perspective

Source: Cutting for Stone