ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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the upper and lower plates of the stage of a Microscope, and heats it by 

 the electric current. To measure the degree of heat, he employs the 

 bi-metallic thermometer (fig. 56). The spiral is made of brass S and iron r 



Fig. 57. 



soldered together, and by 

 the difference of expansion 

 in the two metals the spiral 

 contracts or opens. The 

 inner end a is attached to 

 the stage close to its open- 

 ing, while the free end h 

 acts, through an arm c, on 

 an index d which is pivoted 

 at X, and whose point Ji 

 moves along the scale / g. 

 Or the thermo-electric ap- 

 paratus (fig. 57) may be 

 used, where e e is iron and n 

 German silver, two wires e' 

 and n' leading to the gal- 

 vanometer g, the needle of 

 which is deflected more or 

 less, according to the tem- 

 perature of the stage. 



3. Eot Plates.— M. G. 

 Chevalier's* is shown in 

 fig. 58. It consists of a 

 metal plate with a central 

 aperture, beneath the two 

 ends of which are placed 

 spirit-lamps which slide up 

 and down on the projecting 

 stems. This apparatus was 

 intended for use with Cheva- 

 lier's Universal or Chemical Microscope (a modified form of the latter 

 shown in fig. 58), in which the objective is beneath the object. One or 



* Chevalier,. C, 'Des Microscopes at de leur usage,' 1839, p. 97 (1 pi.)- 



