152 MICROSCOPE. [CH. VI 



Fig. 27 shows the micrometer; it bears at its lower 

 extremity a needle which, as explained above (exp. 155, 

 p. 135), 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. 



The cord from the plant passes round an easily 

 moveable pulley, and ends in a small weight. As the 

 plant grows the pulley turns, and its rotation, magnified 

 by means of an index projecting radially from the pulley, 

 is read off from time to time on a graduated arc. The 

 instrument is described and figured in Sachs' Text-hook 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 Sachs 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. 



