CH. V] POLARISCOPE. 119 



movement which may be recorded in various ways, e.g. 

 by a horizontal microscope, or by the micrometer-screw 

 described below, experiment 155. The weight will com- 

 press the dry wood slightly, so that it is necessary to 

 wait until the index comes to rest before the water is 

 added. The experiment is a rough one, the unsteadiness 

 of the weight on the small bit of wood being a source of 

 error, another being introduced by the unequal swelling of 

 the wood tipping the weight slightly on one side. For 

 this reason we use the following arrangement. The 

 ordinary 28 lb. weights are pierced by a hole so that the 

 surface of the wooden block is visible from above. It is 

 therefore possible to use as an index a vertical wire or a 

 glass filament, the lower end of which rests in a shallow 

 depression in the surface of the wood, while it is kept 

 vertical by passing through a hole in a fixed metal plate 

 or horizontal loop of wire. The movement of the upper 

 end of the glass filament is then observed with a horizontal 

 microscope. 



(140) Observations luith the polariscope. 



The appai'atus consists of two parts, the polariser and 

 the analyser. In the Zeiss pattern of instrument, the 

 former of these is to be fixed axially on the substage of 

 the microscope above the mirror; the analyser separates 

 into two pieces, one, a disc — which should be graduated — 

 fastens on to the upper end of the tube like a collar ; into 

 this an ordinary ocular is slipped, and the other piece is 

 fitted on to it like a cap. 



The essential part of each is a " Nicol's prism " — 



