200 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



Polarization by bending a Glass 

 Tube. (Figs. 726 and 727.) 

 Analagons effects are produced by 

 slightly bending a piece of anneal- 

 ed glass in a suitable frame. To 

 illustrate this, a slip of glass about 

 six inches long, half an inch wide, 

 and one-third of an inch thick, is 

 confined in a suitable brass frame, 

 and pressed at two points by a steel bar, having a screw by 

 which any required pressure may be given ; the glass may 

 thus be unequally and differently strained ; the minute parts 

 on the convex side are urged asunder and their attractive 

 forces called into operation, while tho^e on the concave sur- 

 face are pressed together, and tin-it- repulsive forces brought 

 into action. Between these t\vo oppositely affected surfaces 

 there is a neutral line where equilibrium exists, and on both 

 sides of this the degree of strain augments as we recede 

 from the line. Now, if the slip of glass be examined while 

 in the polariscope, it will be found to have acquired while 



Fig 727. 



in the bent state, double refracting properties. Two sets of 

 colored fringes are perceived, one on the convex or dilated 

 side of the plate, and the other on the concave or compress- 

 ed side. Between these two sets of fringes is a black line, 

 indicating the situation where neither compression nor dila- 

 tion exists, and where, therefore, double refraction is absent. 



Price, $5.50. 



Double Refraction by unequal Heating. To illustrate this, 

 a plate of glass, about 1 inch square, and | of an inch thick, 

 is fitted into a brass frame, having a rod and handle. The 

 frame is heated, and the plate of glass placed therein. 

 Glass being a bad conductor of caloric, and when a heated 

 body is applied to it, the part in contact with this becoming 

 hot, expands, but owing to the bad conducting quality of the 



