Human Frame, 25 i 



the glassy humour) they are all converg- 

 ed and united on the retina ; -the impres- 

 sion thereof being communicated to the 

 common sensory of the brain by the optic 

 nerves, doth there present to the mind 

 the species and image of the object. 



That, which immediately affects the 

 sight, and produces in us that sensation, 

 which \ve call seeing, is light. 



Light may be considered either, first, 

 as it radiates from luminous bodies di- 

 rectly to our eyes ; and thus we see lu- 

 minous bodies themselves, as the sun, or 

 a flame, &c. or, secondly, as it is reflect- 

 ed from other bodies ; and thus we see 

 a man, or a picture by the rays of light 

 reflected from them to our eyes. 



Bodies, in respect of light, may be di- 

 vided into three sorts ; first, those that 

 emit rays of light, as the sun and fixed 

 stars ; secondly, those that transmit the 

 rays of light, as the air ; thirdly, those 

 that reflect the rays of light, as iron, 

 earth, &c. the first are called luminous ; 

 the second pellucid ; arid the third opaque. 



Opaque bodies are of two sorts, specu- 

 lar, or not specular. Specular bodies, or 

 mirrors ? are such opaque bodies whose 



