OPTICS. 101 



With these true principles was mixed much error and indistinctness, 

 even in the best writers. Euclid, and the Platonists, maintained that 

 vision is exercised by rays proceeding from the eye, not to it ; so that 

 when we see objects, we learn their form as a blind man would do, by 

 feeling it out with his staff. This mistake, however, though Montucla 

 speaks severely of it, was neither very discreditable nor very injurious ; 

 for the mathematical conclusions on each supposition are necessarily 

 the same. Another curious and false assumption is, that these visual 

 rays are not close together, but separated by intervals, like the fingers 

 when the hand is spread. The motive for this invention was the wish 

 to account for the fact, that in looking for a small object, as a needle, 

 we often cannot see it when it is under our nose ; which it was con- 

 ceived would be impossible if the visual rays reached to all points of 

 the surface before us. 



These errors would not have prevented the progress of the science. 

 But the Aristotelian physics, as usual, contained speculations more 

 essentially faulty. Aristotle's views led him to try to describe the 

 kind of causation by which vision is produced, instead of the laws by 

 which it is exercised ; and the attempt consisted, as in other subjects, 

 of indistinct principles, and ill-combined facts. According to him, 

 vision must be produced by a Medium, — by something between the 

 object and the eye, — for if we press the object on the eye, we do not 

 see it ; this Medium is Light, or " the transparent in action ;" darkness 

 occurs when the transparency is potential, not actual ; color is not the 

 " absolute visible,'' but something which is on the absolute visible ; 

 color has the power of setting the transparent in action ; it is not, 

 however, all colors that are seen by means of light, but only the proper 

 color of each object ; for some things, as the heads, and scales, and 

 eyes of fish, are seen in the dark ; but then they are not seen with 

 their proper color." 1 



In all this there is no steady adherence either to one notion, or to 

 one class of facts. The distinction of Power and Act is introduced to 

 modify the Idea of Transparency, according to the formula of the 

 school ; then Color is made to be something unknown in addition to 

 Visibility ; and the distinction of " proper" and " improper" colors is 

 assumed, as sufficient to account for a phenomenon. Such classifica- 

 tions have in them nothing of which the mind can take steady 

 hold; nor is it difficult to see that they do not come under those 



De Anim. ii. 6. 



