THE PHENOMENA OF MOTION. 



253 



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

 distinguish this in spherical or cylindrical bodies which revolve 

 Tound 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 perceiving 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, &c. ; we must be able to distinguish clearly which 

 are the sides of the windings turned towards or turned away 

 from us. 



If the course of the windings is very irregular, as in Fig. 139, a 

 little practice and care are needed to distinguish a 

 spiral line, as such, in small objects. The microscopic 

 image might easily lead us to the conclusion that we 

 were examining a cylindrical body composed of bells 

 or funnels inserted one in another. The spirally- 

 thickened threads, for instance, as they originate from 

 the epidermis-cells of many seeds, were thus inter- 

 preted, although here and there, by the side of the 

 irregular spirals, quite regular ones are also observed. 



Moreover, it must not be forgotten that in the 

 microscopic image a spiral line always appears wound 

 in the same manner as when seen with the naked 

 eye, whilst in a mirror (the inversion being only a 

 half one) a right-handed screw is obviously repre- 

 sented as left-handed, and conversely. If, therefore, the micro- 

 scopic image is observed in a mirror, as in drawing with the 

 Sommerring mirror, or if the image-forming pencils are anywhere 

 turned aside by a single reflexion, a similar inversion takes place 

 from right-handed to left-handed, and this inversion is again can- 

 celled by a second reflexion, as in Oberhseuser's camera lucida, and in 

 many multocular Microscopes. All this is, of course, well known, 

 and to the practised observer self-evident ; nevertheless, many 

 microscopists still appear to be entirely in the dark about matters 

 of this kind. 



FIG. "139. 



