CH. Vl] MICROSCOPE. 139 



meter, between it and the light, and the moment of 

 contact of the needle-point and the mercury determined 

 by the distortion of the image of the thread seen by 

 reflection in the mercury. For this method the apparatus 

 must stand on a steady table. Fig. 25 shows the micro- 

 meter; it bears at its lower extremity a needle which, as 

 explained above (exp. 155, p. 123), is useful for various 

 measurements. The hook on the edge of the cup gives a 

 means of knowing when the oil in the cup is horizontal. 

 If this is the case the point of the hook will remain in a 

 constant relative position with regard to the surface, as 

 the screw is turned. Thus if the cup is filled so that the 

 point just dimples the surface, that state of things should 

 be continuous during rotation. 



(179) Arc-indicator. 



This instrument is described and figured in Sachs' 

 Text-Book of Botany. 2nd edit. Engl. Tr. p. 826. 



(180) Microscope. 



The special merit of the microscopic method is that by 

 its use the attachment of a cord to the plant is rendered 

 unnecessary. The plant therefore grows in normal con- 

 ditions, moreover it is now possible to keep the plant in 

 constant slow rotation about a vertical axis, and thus to 

 avoid heliotropic curvatures ; lastly by this means delicate 

 plants, e.g. moulds, and delicate plant-members, e.g. roots, 

 can be observed^ We use a horizontal microscope 



^ The microscopic method as designed by Saehs is described and 

 figured in Vines' paper in Sachs' Arbeiten, ii. p. 135. The horizontal 

 microscope may of course be used to read the descent of the weight in 

 experiments of the type of 177, 178. 



