OPTICAL INSTRUMENTS. 



201 



one 



medium, the surrounding Fi s 728 



parts, not being influenced 

 by the heat, do not expand, 

 but resist the dilation of the 

 heated portion. In this way, 

 therefore, the immediate ef- 

 fect of heat on one part of a 

 piece of glass (Fig. 728) is 



to put all the surrounding parts into a strained state ; 

 part is expanding, and the other parts are resisting the dila- 

 tion. It is obvious, then, from the unequal states of tension 

 of the different parts of a piece of glass thus partially heated, 

 that it ought to acquire doubly refracting properties ; and 

 the polariscope shows that it does so. In this state, the 

 glass exhibits distinct neutral and doubly refracting axes. 

 Unequal cooling will also cause the plate of glass to acquire 

 doubly refracting properties, and when examined by polar- 

 ized light, presents fringes, &c., similar to those observed 

 in glass unequally heated. Price, $2.00. 



Uhannealed Glass a Double Refractor. On considering the 

 mechanical condition of unannealed glass, it is obvious that 

 the different parts are unequally and differently strained ; 

 the internal being in a state of distension, the external in 

 that of compression. So that the state of the different parts 

 and the distribution of the forces will be exactly similar to 

 those described in annealed glass, occasioned by unequal 

 heating. One very important point in reference to these un- 

 annealed glasses is that the polarizing structure depends 

 entirely on the external form of the glass plate, and on the 

 mode of the aggregation of its particles. This will be very 

 obvious by observing the different shapes of the fringes 

 respectively presented by square, circular, oval, triangular, 

 rectangular, and other shaped plates (Fig. 729). The cir- 



Fig. 729. 



cular and square plates have only one axis, whereas the 

 oval and rectangular have two axes. These unannealed 

 glasses are of a variety of shapes, and are usually about one 



