Dioptrics

...does not escape refractions: for they are effects of the air, not of fire. They establish, therefore, the denser surface of the air, as was said above; but they utterly overthrow and eliminate the thinner region of fire, as Pena Jean Pena (1528–1558) was a French mathematician who argued against the Aristotelian idea of a "sphere of fire" surrounding the Earth. wishes. Therefore, from both sides, the excellence of Optical demonstrations shines forth, as much in establishing the distinction of the air from the ether ether: the substance then believed to fill the heavens above the atmosphere., as in removing the fictitious sphere of fire In ancient and medieval physics, the "sphere of fire" was a layer of burning element supposed to exist between the Earth's atmosphere and the Moon..

In the fifth place, Pena indicates how much ignorance holds physicists regarding the matter, location, and effects of Comets, unless they have entered the schools of Optics; and what this discipline teaches concerning such portents of nature to those who do not despise it.

Again, therefore, I recommend this truth to the reader: that the bodies of Comets (whether they be "hairy," "bearded," or "tailed" stars) are shown by Optics to be clearly transparent, based on the argument that they keep their tails turned away from the Sun.

Secondly, it is also true that those transparent bodies are denser than the ether in which they travel. It is also a third truth that, from the analogy of the motion of comets, we are taught much about the location of Comets, and we hold it certain that most of them move above the moon in the highest ether. But the fourth point that Pena adds is doubtful: whether a power of heating exists in Comets by the law of Optics, while the refracted rays of the sun—upon entering and exiting the body of the comet—are forced together into the sharp point of a cone original: coni mucronem. Kepler is describing a "caustic" or focal point where light rays intersect after passing through a lens-like body. behind the body, and by this gathering together they conceive a power of igniting. For even if the rays do come together into a cone in this way, the violence of inflammation would never follow except at that very point of the cone, in the depths of the ether. What, truly, does this have to do with that heat which is stirred up here on Earth? Furthermore, that visible tail of the Comets is not the cone of rays itself using the body of the comet as its base; but if we give much weight to this speculation, this tail is a new cone, beginning there where the other cone (whose base is in the body of the comet) ends in a point: which the law of optics teaches happens very close behind the body of the comet. Therefore, the rays of the sun, insofar as they constitute that visible track which we call the tail, are now diverging again. Indeed, igniting arises not from the divergence, but from the intersection of rays. Therefore, there is no power of igniting in the tail, but if there is any, it is in the intersection...