CH. XIV] REFLECTION AND REFRACTION 573 



the point of reflection (fig. 309). Most cases of irregular and 

 semi-regular reflection if considered from the standpoint of a small 

 enough part of the surface are really cases of regular reflection; 

 that is, any small particle of which the surface is made reflects the 

 light striking it regularly, but each particle of the surface reflects 

 the light in a different direction. Hence, taken as a whole, such a 

 surface will not reflect the light regularly. (See Mirrors 800). 



795. The use of regular or mirror reflection in projection is 

 illustrated by the mirrors used in opaque lanterns (fig. 95-1 10) and 

 by the mirrors and prisms used with drawing apparatus (fig. 180- 

 204). 



With regular or mirror reflection the observer can only see the 

 light when he is in the path of the rays either before or after the 

 reflection ( 796a). 



796. Irregular reflection. If the surface is irregular then 

 the light striking it is reflected in various directions depending upon 

 the position of the irregularities on the surface receiving the light. 

 If these are very small, as in dust or smoke or as on the surface of 

 snow, white cloth or paper, etc., then the reflected light is scattered 

 practically equally throughout the entire hemisphere toward 

 which the surface faces (fig. 310). 



797. The use of irregular reflection is illustrated by the 

 reflected rays from the white screen upon which the image is pro- 

 jected by the magic lantern, pro- 

 jection microscope, etc. The im- 

 age appears almost equally bright 

 from any point in the room. 



796a. If there is dust, smoke or 

 fog in the path of the beam of light either 

 before or after reflection, the minute 

 particles in the smoke or fog irregular- 

 ly reflect some of the light and one can 

 see it at any angle (fig. 320-323). Dust 

 or scratches on the surface of themir- FlG - 3- IRREGULAR OR DIFFUSE 

 ror enables one to see where the beam REFLECTION. 



of light strikes its surface. If there is A ray of light striking a rough sur- 

 no irregular reflection then one can only face is scattered equally in all direc- 

 see a beam of light when in its path. tions. 



