374 



Sir J. C. Bose and Mr. G. Das. 



4 seconds is 38 mm. in tlie first record. It is precisely the same in the 

 record taken 15 minutes later. The successive growth-elongations at 

 intervals of 1 second are practically the same throughout, being 9"5 mm. 

 This uniformity in the spacings demonstrates not only the regularity of 

 growth under constant conditions, but also the reliabihty and perfection of 

 the apparatus. It also shows that by keeping the external conditions con- 

 stant, the normal growth-rate could be maintained uniform for at least 

 15 minutes. As the magnified growth is nearly 1 cm. per minute, and as it 

 is quite easy to measure 0*5 mm., the crescograph enables us to record a 

 length of 0"00005 mm., that is to say, the sixteenth part of a wave of red 

 light. The absolute rate of growth, moreover, can be determined in a period 

 as short as 0"05 second. These facts will give us some idea of the enormous 

 possibilities of the crescograph for future investigation. 



As the period of the experiment is very greatly shortened by the method 

 of high magnification, 1 shall, in the determination of the absolute rate of 

 growth, adopt a second as the unit of time and fi or micron as the unit of 

 length — the micron being O'OOOOOl metre, or O'OOl mm. 



If m be the magnifying power of the compound lever, and I the average 

 distance between successive dots in millimetres at intervals of t seconds, 

 then 



rate of growth = —^y. 10^/a per sec. 



In the record given I = 9'5 mm., m = 10,000, t = 1 second. 

 Hence rate of growth 



9'5 



= X 10^/i per sec. = 0'95 /i per sec. 



Expt. 2. Precaution against Physical Disturbances. — There may be some 

 misgiving about the employment of such high magnification; it may be 

 thought that the accuracy of the' record might be vitiated by physical 

 disturbance, such as vibration. In physical experimentation far greater 

 difficulties have been overcome, and the problem of securing freedom from 

 vibration is not at all formidable. The whole apparatus need only be placed 

 on a heavy bracket screwed on the wall to ensure against mechanical 

 disturbance. To what extent this has been realised will be found from the 

 inspection of the first part of the record in fig. 5, c, taken on a moving plate. 

 A thin dead twig was substituted for the growing plant, and a perfectly 

 horizontal record not only demonstrated the absence of growth-movement 

 but also of all disturbance. There is also another element of physical change, 

 against which precautions have to be taken in experiments on variation of 



