DETERMINATION OF CRITICAL POINT OF DEATH 165 



would rotate round a vertical axis passing through the centre 

 of the spiral, under the influence of the differential expansion 

 or contraction produced in the compound strip of metal by 

 rise or fall of the temperature. When the outside of the 

 circumference of the spiral consists of the more expansible 

 metal, brass, then a rise of temperature will produce the 

 movement of curling. The difficulty in the construction of 

 this part of the apparatus lies in securing equal angular 

 rotation of the diameter about the axis, with every equal rise 

 of temperature. These indications 

 were at first extremely irregular. 

 I was able, however, to remove all 

 traces of irregularity by careful 

 and repeated annealing. In any 

 case the thermometric indications 

 of the compound helix may be 

 previously calibrated. 



The axis of the Optic Lever — 

 one arm of which is attached to 

 the plant-specimen, and which is 

 to give the record of its variation 

 in length with rise of temperature 

 — is now supported on the diameter 

 of this last half-circle of the helix 

 and is thus rotated bodily, with rise 

 of temperature (fig. 83). And it 

 will thus be seen that the motion 

 of the spot of light, reflected from 



the single mirror of the Optic Lever, is a resultant of two 

 movements, which take place at right angles to each other 

 — namely, the horizontal movement, due to thermometric 

 variation and the corresponding vertical movement, due to 

 the changes of length of the experimental plant-tissue. 

 Owing to the fact that the spot of light in this apparatus is 

 reflected only once, it is extremely bright. 



I shall now proceed to describe the manner in which the 

 plant is mounted, and other accessories of the apparatus. 



Fig. 83. The Thermometric 

 Spiral and Optic Lever of 

 the Morograph 



