152 



OPTICS. 



Lesoon X 



115. T. What is meant by catoptrics? 

 /'. It is that part of the scienc-- of optics 



which treats of the laws of refl, cted light, 

 and the phenomena of vision produced 

 by reflection. The term is derived from 

 the Greek word kutoj>tron (*cdronrpor), 

 a mirror. 



116. T.lf a ray of 1'ght be admitted 

 ii.to a darkened room, and allowed to fall 

 upon a polished metallic surface, what 

 will be the effect? 



P. It will he returned or reflected in 

 such a manner thar the angle of incidence 

 shall be exactly equil to the angle of re- 

 flection. For example, if n i (/*/(/. 31) he 

 the direction of the inci- 

 </v M m dent ray, and i p a per- 

 pendicular drawn from 



the surface of the mirror 



* ni m m, the ray will be re- 

 Fiff. 31. fleeted in such a direc- 

 tion t d, tnat the angle of reflection dip 

 is equal to the angle of incidence n i p. 

 Hence it is clear that the ray makes the 

 s.iine angle with the perpendicular, both 

 before and after its reflection. 



117. T. What is the relative position 

 of the image of an object when seen in a 

 reflecting I'lane? 



1'. -It is such that every part of the 

 image will appear as far behind the plane 

 as the object itself is before it. [Experi- 

 Take a mirror (A B Fig. 32), 

 and hold any object 

 (such as a knife, pen, or 

 arrow), in the position 

 shown in F H in this 

 figure, and you wll see 

 the image of the obj. ct 

 F iff. 32. reflected, as E c. in the 



annexed figure. Now if we examine the 

 diagram, we shall find that the lint s F c 

 an'i H D are equal to the lines c E and 

 D G. and therefore the intake appears as 

 Jar behind the plane of the mirror as the 

 object before it. 



118. T. Does the position of the 

 mirror alter the position of the image of 



the ol ject reflect* d in it? 

 P. Yes. [Experi- 

 ment 19.] You obser e 

 that I ha^e H pencil here 

 (A B, Fi<j. 33 ) which is 

 plaeed perpendicularly, 

 Fig. 33. and I now place a inir- 



QUEST10NS. 



rueut 18.J 



ror (c) at an angle of about forty -live 

 degrees, with its reflecting surface 

 towards the pencil ; now if you look 

 in the mirror yon will observe that the 

 image of the pencil is horizontal instead 

 of being perpendicular. [Looks in the 

 mirror, and observes the image to be in 

 the same position as the arrow a it, Fiy. 

 33.] Now I will reverse the position of 

 the pencil and place it horizontally, and 

 you will then observe that the image 

 appears to be perpendicular. [Does so.] 



119. T. Why do we observe the 

 reflection of our figure in a mirror re- 

 cede when we recede, and approach as we 

 approach the mirror? 



P. Because the lines and angles of 

 incidence are always equal to the lines 

 and angles of reflection. [See Q. 114.] 



120. T How can you observe the pro- 

 file of your face? 



P. By standing between two mirrors 

 s* placed thus 



f' 1 



and 

 ing 



Fig. 34. 



look- 

 at A, so 

 that a three- 

 quarter view of 

 the face is ob- 

 served, then 

 turning the 

 eyes towards 

 you will ob- 

 serve your pro- 

 file distinctly 

 reflected in the 



mirror. By altering the position of the 

 mirrors the reverse effect, is obtained. 



121. T. How do you account for the 

 profile of your face being observed? 



P. Because the image which is de- 

 lineated behind one mirror serves as an 

 object to be reflected from the surface of 

 the other. 



122. 7'. Upon what principle is the 

 kaleidoscope constructed ? 



P' The multiplication of the reflec- 

 tion of an object 

 caused by placing it 

 between two mir- 

 rors inclined to- 

 wards each other at 

 any angle. For ex- 

 ample, here is a dia- 

 gram showing two 

 Ft'}. 35. mirrors inclined to- 



