[ 200 ] [Book III, 



CHAP. IV. 



GENERAL VIEW OF THE PHENOMENA 

 O F R E F R A C T I O N. 



Laws of Refraction. Degree of Refraftion which Light fujfers front 

 Jiffertnt Mediums." Common Phenomena of Refraction. Refraflioit 

 by fpherical Surfaces By convex Surfaces By concave Surfaces, 

 OfLenfe s. Convex Lenfes. "Concave Lenfes. Different Refran- 

 gibilitj of the Particles of Light. Experiment with the Prifm. 



IT has been proved that light, like every known 

 fubftance, is fubject to the laws of attraction; it 

 has been intimated too, that even its propenfity to 

 move in a direct line is, in certain cafes, overcome by 

 this fuperior influence; and that the direction of the 

 rays of light is changed in paffing from one medium 

 to another. The fpace in which a ray of light moves 

 is called a medium, whether pure fpace, air, water, 

 glafs, or any other tranfparent fubftance ; and when a 

 ray is bent out of its natural courfe in paffing from one 

 medium to another, it is faid to be refrafted or broken, 

 probably from the broken appearance which a ftaff, 

 &c. exhibits when part of it is immerfed in water. 



There are two circumftances eiTential to refraction ; 

 i ft, That the rays of light mall pafs out of on-e me- 

 dium into another of a different denfity, or of a greater 

 or lefs degree of refiftance. 2dly, That they pafs in 

 an oblique direction. 



The denfer the refracting medium, or- that into 

 which the ray paffes, is, the greater will be its refract- 

 ing power j and of two refracting mediums of the fame 



denfity, 



