The Summary of Physics Restored

205Every transparent body, especially glass, is an intermediary of radiance, because it receives radiance within itself and communicates what it receives to the air beyond. It does not do this by transmitting luminous air from one side to the other, which is contrary to Nature, but by two other means. First, a transparent body is open to the passage of the spirit of radiance, receives it within itself and transmits it. Once transmitted, the spirit enters the air, whence comes abundant radiance. Second, through the benefit of the radiance it receives, the transparent intermediary becomes not merely illuminated but luminous, kindled with radiance like a wick by the spirit of light, which is friendly to transparent things. Every luminous body has the right and capacity to pour out radiance, something dense and opaque bodies cannot do except by reflection.