OPTICS. 107 



tucla speaks severely of it, was neither very dis- 

 creditable 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 

 conceived would be impossible if the visual rays 

 reached to all points of the surface before us. 



These errours would not have prevented the pro- 

 gress of the science. But the Aristotelian physics, 

 as usual, contained speculations more essentially 

 faulty. Aristotle's views led him to try to de- 

 scribe the kind of causation by which vision is pro- 

 duced, instead of the laws by which it is exercised ; 

 and the attempt consisted, as in other subjects, of 

 indistinct principles, and ill-combined facts. Ac- 

 cording 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 trans- 

 parent in action ;" darkness occurs when the trans- 

 parency is potential not actual ; colour is not the 

 " absolute visible," but something which is on the 

 absolute visible ; colour has the power of setting 

 the transparent in action; it is not, however, all 

 colours that are seen by means of light, but only 



