140 REVOLVING DRUM. [CH. VI 



designed by Mr H. Darwin of the Cambridge Scientific 

 Instrument Company, which has a wide field, a long focal 

 distance and reads to O'l mm. ; also one by Albrecht 

 of Tubingen, whose lowest power gives 0'044 mm. for 

 each division of the eye-piece micrometer, with a focal 

 distance of 6 cm. and a field of 4 mm. in diameter. The 

 microscope can be raised and lowered by a micrometet 

 screw ; and can, by another screw, be slowly rotated on the 

 vertical axis, — a movement which is very convenient. 



(181) Self-recording auxanometer. 



This instrument is described and figured by its in- 

 ventor, Sachs, in his Arbeiten^. The principle is well 

 known, namely, that the descent of the weight, magnified 

 by means of a lever, is recorded on a drum rotating on a 

 vertical axis. In Sachs' paper, p. 116, is an interesting 

 facsimile of the smoked paper on which a plant has 

 written its hourly growth. 



Sachs uses a drum with continuous motion which, 

 from being excentric about its vertical axis, only comes in 

 contact with the index at regular intervals of time, for the 

 rest of the time the index is free from all constraint, and 

 this is an advantage. The drum which we use was de- 

 signed by Mr H. Darwin^ The drum (shown in fig. 26) 

 measures 25 cm. in height and 512 mm. in circumference : 



' Vol. I. p. 113, also Text-Booh of Botany, Engl. Tr. Ed. li. pp. 

 827-28. 



^ It is on the same general principle as that of Baranetzky, (see Vines' 

 Physiology, p. 399). It was however invented independently of this 

 naturalist : we have in the Cambridge Laboratory a drum constructed by 

 Mr H. Darwin about 1876. 



