METHOD OF OPTICAL MAGNIFICATION 383 



mirror, fixed at the upper end of the stem, reflects a spot of 

 light on to a scale which may be placed at a distance 

 of 3 metres, the scale itself being divided into millimetres. 

 The magnification thus produced is then 6000 times, which 

 is sufficient to measure the rate of torsion in the course 



Fig. 217. Method of Optical Alagnification. 



P, stem, upper end of which is held in clamp c (shown in the side 

 figure); M, plant-mirror; m', fixed mirror; s', scale for 

 measurement of torsion. Plant held at lower end in a 

 small phial filled with water. 



of a minute. Certain precautions have to be taken in attach- 

 ing the string to the tip of the plant. The very crude method 

 of tying a knot gives rise to unequal pressure on different 

 sides of the organ, resulting in the transmission of unequal 

 excitations down the stem. To avoid this, cotton-wool is 

 wrapped round the tip, to act as a soft cushion. A light 

 three-pronged aluminium clamp now holds the tip of the 

 plant. A spring clip attaches the plant-mirror to the clamp 

 (fig. 217). 



