determination of Reflection and Refraction. 333 



0° to 90°, as shown in the diagram, the diameter connecting 

 the two zero points being perpendicular to the face of the 

 hemi-cylinder. From a horizontal slit in front of the lantern 

 a beam is sent through the middle of the glass and focussed on 

 the zero-point at the further edge of the card-board, whose 

 plane has been slightly inclined so that the path of the beam is 

 sharply denned upon' it. The incident, reflected, and trans- 

 mitted beams are in the same line, the angle of incidence being 

 zero. 



The hemi-cylinder is now rotated through any desired angle, 

 for example 50°, as shown in fig. 1. The card-board moves 



with it. The room should be but slightly darkened, so that 

 there may be no difficulty in reading the graduation on the 

 circle. Part of the beam is reflected and part refracted. Both 

 are plainly seen and the angles of reflection and refraction are 

 measured. Varying the angle of incidence from 0° to 90°, 

 one readily observes that the ratio of the reflected light to the 

 refracted light decidedly increases. Turning still further, the 

 beam from the lantern strikes normally on the curved surface 

 and is totally reflected within the glass at its plane face (fig. 2). 

 Rotating still further, the re-appearance of the refracted beam 

 announces the critical angle, which is read upon the circle. 



In the common form of apparatus where a beam is deflected 

 by a mirror, then sent through smoky air and cloudy water, 

 new adjustments are necessary for every variation of the angle 

 of incidence, involving some trouble and loss of time. With 

 the hemi-cylinder but one easy adjustment is needed for all. 

 The higher refractive index of glass is an additional advantage, 

 aside from the superior definition. The expense is scarcely 

 more than that of having the hemi-cylinder made by a practi- 

 cal worker in glass. A small silvered mirror is substituted for 

 the hemi-cylinder if the law of reflection alone is to be 

 exhibited. 



