PART IV. clxxxiii
VOL. II. PAGES
separately. Species in fact act in this respect as do their agents, and as do their completed effects. Alhazen thus explains the unity of impression resulting from species of an object entering the two eyes; they are united in the brain. But if species are thus mixed, how do we recognize various objects as distinct? Because through the point of mixture some rays come perpendicularly from the object to the eye, others come to this point obliquely; these latter are eclipsed by the former. We may remark further that the various parts of the agent arc always active: the generation of the specics is therefore continuous. Species unrelated to one another do not commingle; they exist separately and simultaneously " 511-516
PART IV.
CHAPTER I.
We have now to consider how and why species are weakened in their transit. Is there resistance in the medium? There is. For every agent produces rarefaction and condensation, as the first process in every natural action. What is rare resists condensation, and con- versely. Therefore every medium offers resistance. Further, there is a practical limit to rarefaction. It may be said that resistance is only offered by contraries, and that, to many agents generating species, nothing is contrary, in the sense in which darkness is contrary to light. But the resistance is not to the species themselves, but to the processes of rarefaction or condensation which accompany them. 517-518 It is obvious that species, so long as they continue to radiate, and are not arrested by an accumulation of such material as will intensify their action (as in the case of fire acting on fuel), are weakened by distance. The senses are less affected by a distant than a near object. Two causes may be assigned. It is said to be a law that, as the first species is weaker than the agent, so the second is weaker than the first, and so on. This, however, is not invariable. If iron he held at some distance from a magnet, the iron is more affected than the air between them. So a ray, passing through coloured glass, produces a stronger effect on a distant wall than on the intermediate air. Another reason may be assigned. The agent radiates force in all directions. Much force is dispersed thus. And though, for special reasons, the force in any one line may be found not to grow weaker, yet it will be so where the medium is ordinarily pervious. Thus both distance and radiant dispersion account for the weakening of species.
