INTERPRETATION OF MICROSCOPIC MOVEMENT 433 



rapidity of movement, and kind of movement, which living and motile 

 forms effect. 



The observation of the phenomena of motion under the microscope 1 

 has led to many false views as to the nature of these movements. 

 If, for instance, swarm-spores are seen to traverse the field of view 

 in one second, it might be thought that they race through the water 

 at the speed of an arrow, whereas they in reality traverse in that 

 time only a third part of a millimetre, which is somewhat more than 

 a metre in an hour. It must not, therefore, be forgotten that the 

 rapidity of motion of microscopical objects is only an apparent one, 

 and that its accurate estimation is only possible by taking as our 

 standard the actual ratio between time and space. If we wish, for 

 the sake of exact comparison, to estimate the magnitude of the mov- 

 ing bodies, we may always do so ; the ascertainment of the real 

 rapidity remains, however, with each successive motion, the princi- 

 pal matter. 



If a screw-shaped spiral object, of slight thickness, revolves on 

 its axis in the focal plane, at the same time moving forward, it 

 presents the deceptive appearance of a serpentine motion. Thus it 

 is that the horizontal projections of an object of this kind, corre- 

 sponding to the successive moments of time, appear exactly as if the 

 movement w r ere a true serpentine one. As an example of an appear- 

 ance of this nature we may mention the alleged serpentine motion 

 of Spirillum and Vibrio. 



Similar illusions are also produced by swarm-spores and sperma- 

 tozoa ; they appear to describe serpentine lines, while in reality they 

 move in a spiral. It was formerly thought that a number of differ- 

 ent appearances of motion must be distinguished, whereas modern 

 observers have recognised most of them as consisting of a forward 

 movement combined with rotation, where the revolution takes place 

 sometimes round a central, and sometimes round an eccentric, axis. 

 To this category belong, for instance, the supposed oscillations of 

 the oscillatorice, whose changes of level, when thus in motion, were 

 formerly unnoticed. 



In addition to these characteristics of a spiral motion it must, of 

 course, be ascertained whether it is right- or left-handed. To dis- 

 tinguish this in spherical or cylindrical bodies, which revolve round 

 a central axis, is by no means easy, and in many cases, if the object 

 is very small and the contents homogeneous,' it is quite impossible. 

 The slight variations from cylindrical or spherical form, as they 

 occur in each cell, are therefore just sufficient to admit of our per- 

 ceiving whether any rotation does take place. The discovery of the 

 direction of the rotation is only possible when fixed points whose 

 position to the axis of the spiral is known can be followed in their 

 motion round the axis. The same holds good also, mutatis mutandis, 

 of spirally wound threads, spiral vessels, itc. ; we must be able 

 to distinguish clearly which are the sides of the windings turned 

 towai-ds or turned away from us. 



If the course of the windings is very irregular, as in fig. 367, a 

 little practice and care are needed to distinguish a spiral line as 



1 Das Mikroskop, Naegeli and Schwendeiier, p. 258 (Eng. edit.). 



P F 



