For Roger Bacon the “forms” that were transmitted towards an observerʼs eyes were analogous to alchemical “seeds”.
It was not exactly like that: each point belonging to a luminous object sends the “species” in the form of multiplication. Species in Latin means form-semblance-image-model.
According to Bacon, the rays emanating from every object do not require light but propagate even in dark condition, to create an effect on another object. So, given Bacon’s theory on rays propagation, does it make sense then to divide between light and darkness?
From the point of view of Roger Bacon’s physics, it makes no sense to divide between light and darkness, since the medium of propagation is unchanged.
Still, according to Bacon, the rays emanating from the stars can only operate through light…
The rays emanating from the stars can only operate through light. Whether it was visible or invisible light Bacon did not say, but he suggests that it was a light that went beyond the visible spectrum.
… to know, in the end, that Bacon’s rays do propagate in a spherical shape.
Roger Bacon states that the multiplication rays of the “species” propagated in a spherical shape. And, to us moderns, this propagation model resembles the spherical propagation of sound.
Roger Bacon discusses extensively the interactions between the macrocosm and the microcosm—that is, between man and the cosmos; for Bacon, this interaction occurs primarily through vision, specifically through the production of images. If I recall correctly, Al-Kindi went so far as to claim that images—even hand-drawn figures—can emit rays into matter.
For Bacon, the visual power is not only a recipient, but also an agent, a source of species, which ennobles the medium and the species of the visible object, making them capable of stimulating sight. Mind that what in philosophy is called “species” finds its counterpart in Alchemy as “seed”. The context of Bacon’s admission of visual radiation seems to be astrological. He believed that the study of light and its effects brought together and explained how the Microcosmos, that is a human being, and the universe, the Macrocosmos, interact. It’s a bit convoluted to explain: Vision, or the production of images, is the species (alchemical seeds) that reaches the stars, and to vision the species, or images of the stars reach to produce vision (which will produce images). The crux of the matter now is to understand whether Bacon was referring to Vision, that is, the mental function that involves understanding, memory, and interpretation of the world, or Sight, that is to the biological function of the eyes to capture light and focus an image. Very likely, Roger Bacon did not distinguish between the two functions and immediately translated “arriving light = production of images“.
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Main Index: 3. Rays and Optics according to the Ancient Authors