æther

I ſuppose this æther pervades all groſs bodies, but yet ſo as to ſtand rarer in their pores than in free ſpaces, and ſo much the rarer, as their pores are leſs. And this I ſuppose (with others) to be the cauſe, why light incident on thoſe bodies is refracted towards the perpendicular; […] I ſuppose the rarer æther within bodies, and then denſer without them, not to be terminated in a mathematical ſuperficies, but to grow gradually into one another; […]