108 POLARISCOPE. [CH. V 



(140) Observations with the polariscope. 



The apparatus consists of two parts, the polariser and 

 the analyser. In the Zei^s 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 so-called Nicol's prism 

 — pieces of doubly refractive Iceland spar so cut and 

 disposed that the light transmitted is all polarised in 

 one particular axial plane. The central upper part of the 

 analyser rotates on the collar-like disk and bears a pointer 

 which records the amount of the rotation. If, with the 

 parts placed in position and the eye at the ocular, 

 the analyser be rotated until its plane of polarisation 

 becomes identical with that of the polariser, the light will 

 be transmitted through it undiminished and the field of 

 the microscope appear bright. On now slowly turning 

 the analyser either way through a right angle, the light 

 will' gradually fade until the field is completely dark. In 

 this latter position the polarising planes of the two prisms 

 are at right angles and the analyser intercepts all the 

 light transmitted by the polariser. On continuing the 

 rotation of the analyser through a further 90°, maximum 

 brightness from coincidence of the planes will be again 

 obtained. With the dark field, place on the stage of the 

 microscope a slide on which is mounted some anisotropic, 



