104 EDITH BELLAMY SHREVE 



always be done by taking a galvanometer reading with the 

 clip in the air instead of on a leaf. 



Figure 2 shows the various units assembled on a board 

 about 7 inches square. The board is screwed to a photographic 

 tripod and the whole may be easily moved about from plant to 

 plant and the height quickly adjusted to suit the position 

 of the leaf under observation. In order to avoid undesirable 

 thermal currents the entire apparatus must be either in the 

 sun or in the shade. It is almost impossible to avoid shading 

 parts of the instruments with the hands or body of the observer 

 and consequently it is more convenient to provide a shade. 

 A doll's parasol fastened by a clamp to the rod holding the 

 thermos bottle was found useful. The leaf to be measured 

 can remain in the sun until the clip is placed on it. It requires 

 only a fraction of a second for a reading to be made; so no measur- 

 able change in the temperature of the leaf can take place before 

 the reading is made. 



