520 



APPENDIX 



X. OPTICS. 



Demonstration of the artificial eye and the electric lantern, including 

 its lenses, used to illustrate it. Complete diagrams of all the optical 

 conditions here illustrated are to be drawn in the note-books (out of the 

 Laboratory if necessary) for inspection. 



Expt. 91. Reflection by Plane Mirrors. (Apparatus: Round dia- 

 phragm in lamp, plane mirror.) Let a pencil of parallel rays enter the 

 "eye 1 " and fall upon the plane mirror. Rotate the mirror on its vertical 

 and horizontal axes so that its surface may be at various angles to the 

 rays of light. Observe (.4) that the angle of reflection is always equal 

 to the angle of incidence, and (B) that the incident ray, the perpendicular 

 at the point of incidence and the reflected ray are always in the same 

 plane. 



Reflection by plane mirrors. Angle a' always equals angle a, and always lies in the same 

 plane. So far as mere optics are concerned, an eye at A sees B at B', but there are subjective 

 differences which complicate the experience beyond optical explanation. 



Expt. 92. Reflection by Spherical Mirrors. (Apparatus: Concave 

 mirror.) Have no diaphragm in the lantern, but adjust the draw-tube 

 so that the rays entering the eye are parallel. 



(A) Concave Mirrors. Place the concave mirror in the rays and 

 observe the principal focus of the mirror. This is a bright spot 2.5 cm. 

 from the mirror, the latter being a portion of the periphery of a sphere 

 of 5 cm. radius. Turn the mirror at various angles and observe 

 illustrations of principle A of the preceding experiment. Place the 

 round diaphragm in the lamp and note the same facts under somewhat 

 different conditions. 



(B) Convex Mirrors. Remove the diaphragm, and turn the mirror 

 about so that the convex side is toward the light. Note the dispersion 

 of the rays. Draw the diagrams. 



