24 PHYSICS OF STREAMING 



scale projected on to the field. The time was estimated by the aid of 

 a metronome, thus leaving the eye free to follow the particle. Only 

 particles of a certain size and character were observed at a given time, and 

 only the times of those which kept to the marked track were taken. The 

 velocity of the stream varies at different depths, and hence the observa- 

 tions were made under the high power, which was kept focussed as closely 

 as possible to a measured constant depth beneath the non-rotating 

 ectoplasm. A large number of observations (10-20) were taken first on 

 one side of the indifferent line or of the cell, and then on the other, and the 

 mean of these found. 



These results are, however, liable to vary, since the general velocity 

 may alter during the time of observation, and since the velocity is not 

 always uniform throughout. 



The following data were obtained from a cell of Nitella : 



Temperature, I7-5C. ; beats of metronome, 175 in 60 sees. ; distance traversed, 0-275 mm. 



The same relative rates were shown after the lapse of half an hour, 

 and no measurable thickening of the apparently more slowly rotating 

 portion could be noticed, as must have occurred if the velocity of the 

 particles accurately represented that of the fluid plasma. Two conditions 

 for accurate observations are (i) that a constant source of illumination 

 should be used, and the percentage of radiant heat reduced as far as 

 possible ; (a) that an optimal supply of oxygen should be assured. 



The observations are less open to error if the same region is observed, 

 and its position in space altered by tilting the stand until the stage is 

 vertical and rotating the latter until the object is reversed. Successive 

 series of readings may be taken, or the object may be reversed after each 

 observation. 



Nitella (i7.5C). 



By taking alternating sets of successive observations fairly constant 

 results are obtained for each particular cell by observing similar particles. 



1 Under the microscope the directions are of course reversed. 



