CH. Vl] REVOLVING DRUM. 153 



designed by Mr H. Darwin of the Cambridge Scientific 

 Instrument Company, which has a wide field (10 mm.), a 

 long focal distance (about 23 cm.), 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 micro- 

 meter, with a focal distance of 6 cm. and a field of 4 mm. 

 in diameter. The microscope can be raised and lowered 

 by a micrometer 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 1 . 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 instrument which we use was 

 designed by Mr H. Darwin 2 . The drum (shown in fig. 28) 



1 Vol. i. p. 113; also Text-book of Botany, Engl. Tr., Ed. n. pp. 

 827-28. 



2 It is on the same general principle as that of Baranetzky (see the 

 figures in Vines' Physiology, p. 399). It was however invented inde- 

 pendently : we have in the Cambridge Laboratory a drum constructed 

 by Mr H. Darwin about 1876. 



