142 Mayow 



to them, and, by tickling them, cause sneezing. But 

 luminous particles seem to propagate the image of 

 themselves by the very gentle impulse of a peculiar 

 extremely fine medium. And this when conveyed 

 by a continuous undulation of the medium of vision 

 to the eye, and by means of it to the delicate origin 

 of the optic nerves, impresses on them such strokes 

 as are appointed by nature for producing the image 

 and the perception of light. Nor is it necessary for 

 the perception of Hght and colours, as the very acute 

 Descartes pointed out long ago, that a material image 

 resembling the ideas which we form in our minds of 

 objects, should pass from them to the eye, since in- 

 deed, to produce the different sense perceptions in the 

 mind it is only necessary that the fine nerve threads 

 which proceed from the brain should be agitated with 

 various motions. For in this way a blind man who 

 uses a stick to guide his steps, feels and distinguishes 

 well enough, by the various ways in which the stick is 

 moved when it strikes against bodies of all sorts, 

 whether it is a tree, or a stone, or anything else that 

 opposes him. 



As the likeness of a luminous body, so also the 

 image and visible form of an illuminated body appear 

 to be propagated by the motion of a special medium 

 distinct from the action of light. For when the afore- 

 mentioned plane, exposed to the rays of light, is seen 

 by an eye placed in darkness, its visible form is con- 

 veyed to the eye not by reflected rays of light, since 

 these do not reach the eye, but by the motion of a 

 peculiar medium quite different from the action of light. 

 But the mode in which this special medium is moved 

 will be treated of later. 



As to the medium by whose impulse the likenesses 

 of things are propagated, it is probable that it consists 



