400 



REFRACTION OF LIGHT. 



parallel to its direction before entering; the ray merely 

 suffers lateral aberration. Objects seen obliquely through 

 such plates appear slightly displaced from their true position. 



621. Prisms. A prism produces two simultaneous 

 effects upon light passing through it; a change of direc- 

 tion and decomposition. The second of these effects will 

 be considered under the head of dispersion ( 636). 



(a.) Let mno represent a section formed by cutting a prism by a 

 plane perpendicular to its edges. A ray of light from L being re- 

 fracted at a and b en- 

 ters the eye in the di- 

 rection Ic. The object 

 being seen in the direc- 

 tion of the ray as it 

 enters the eye ( 594), 

 appears to be at r. An 

 object seen through a 

 prism seems to be 

 moved in the direction 

 of the edge that sepa- 

 rates the refracting 

 surfaces. The rays FIG. 301. 



themselves are bent 



toward the side that separates the refracting surfaces, or toward 

 the thickest part of the prism. 



(6.) Prisms are generally made of glass, their principal sections 

 being equilateral triangles. In order to give a 

 liquid the form of a prism, it is placed in a 

 vessel (Fig. 302) in which at least two sides 

 are glass plates not parallel. Bottles are made 

 for this purpose. 



(c.} In Fig. 303, ABC is the principal section 

 of a right-angled isosceles, glass 

 prism, right-angled at C. A ray 

 of light falling perpendicularly 

 upon either of the cathetal (cathetus) surfaces, as AC, 

 will not be refracted. With AB, it will make an 

 angle of 45 which exceeds the critical angle for 

 glass ( 617). It will therefore be totally reflected 

 and pass without refraction from the cathetal surface BC. Such 

 prisms are often used in optics instead of mirrors. 



FIG. 302. 



