150 AUXAXOMETERS. [CH. VI 



by watering the plant thoroughly at the beginning of the 

 experiment, and leaving it unwatered during the rest of 

 the observation ; the shrinking of the earth will thus be 

 spread over a considerable period. The most serious error 

 however is the curvature of the stem, which may be either 

 spontaneous or more frequently due to heliotropism. If 

 the plant can be illuminated from above, so much the 

 better ; if not, a large bright mirror must be placed close 

 behind it, which neutralises one-sided illumination. In 

 our experiments we use principally the flower scape of 

 Narcissus, which is but slightly affected by lateral light. 



(177) The descent of the weight measured on a scale. 



The simplest plan is to fix an index to the weight and 

 read its movement on a vertical scale. A piece of sheet- 

 lead 15 mm. x 20 mm. folded across the middle will serve 

 as a weight, and a fine sewing-needle placed horizontally 

 in the fold of the lead can be secured in its place by 

 hammering the lead. In this way the growth can be 

 estimated in O'l mm., and the arrangement might be used 

 for measuring the daily and nightly growth of a plant for 

 a series of days. 



(178) Micrometer screw. 



The weight in this case bears a vertical instead of 

 a horizontal needle, and its descent brings the needle 

 into contact with a surface of oil or mercury contained in 

 a cup which is raised or lowered by means of a micro- 

 meter screw 1 . The moment of contact is a very definite 



1 This method was suggested and the apparatus designed by Mr H. 

 Darwin, of the Cambridge Scientific Instrument Company. 



