GROWTH 



199 



The sensitiveness of the indications of this apparatus 

 depends upon the length of the index. It is incon- 

 venient in many ways to use a very long index ; 

 and therefore, when we wish to have a very sensitive 

 apparatus such as will enable us to observe growth 

 during exceedingly short intervals of time, during one 

 minute, for instance, or, as in the present case, to demon- 



sa 



FIG. 57. 



strate this phenomenon of growth to a large audience, 

 we have recourse to another method. We use some- 

 thing intangible instead of the index a ray of light, 

 to which we can give any desired length without 

 technical inconvenience. For this purpose, instead 

 of the index we fix to the axis of the pulley a small 

 mirror (fig. 57, mn). Light coming from a lamp or a 

 candle placed in front of this mirror will be reflected 

 and form a bright spot somewhere on the wall. The 



