120 POLARISCOPE. [CH. V 



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 disc 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 doubly 

 refractive object such as almost any vegetable tissue, and 

 it will be seen that parts of each of its elements will 

 appear black and parts white. The object thus seen is 

 said to be viewed " with crossed Nicols " and the appear- 

 ance is due to the optical structure of the object so 

 altering the polarisation of the light that part of it and 

 part only is capable of passing through the analyser in its 

 present position. Hence the alternating light and dark 

 markings. On shifting the analyser through a right angle 

 these markings are now seen reversed in their optical 

 properties. 



