168 Dr. Herapath on the Optical Properties of 



If three crystals, a, b and c, arranged as in fig. 8 be examined 

 with one tourmaline, the latter (c) is inferior to the two former, 

 which of course cross it at an angle of 45°, and at 90° to each 

 other respectively. Upon placing a tourmaline over the eye- 

 piece of the microscope at right angles to the plane of c, the 

 phenomenon of polarization will be exhibited by c ; it will appear 

 black. 



The crystals a and b are of course at 45° respectively to both 

 the tourmaline and to c; they are therefore at the angle of de- 

 polarization as in section B,and will consequently exhibit coloured 

 images where they cross the polarizing crystal c, one being com- 

 plementary in colour to the other ; and as they intersect each 

 other at right angles, they there exhibit the appearance due to 

 polarization, and darkness is the result. 



Similar phenomena are exhibited by the hexagonal plates, &c. 



(E.) The next phenomena to be described will be the result 

 of examining these new polarizing and depolarizing crystals by 

 means of two tourmalines, a polarizing and an analysing plate as 

 they are commonly called ; in fact, they would be submitted to 

 the ordinary arrangement of the polarizing microscope. 



Select two crystals superimposed and crossing at right angles, 

 and the whole object capable of being revolved horizontally on 

 its own axis. 



Let the crystals as a cross coincide with the planes of the 

 tourmalines. (Fig. 9, a and b.) 



The field of the microscope will be dark, as the tourmalines 

 are at right angles, and consecpiently nearly the whole of the 

 incident light will be obstructed or polarized. 



The crystal (a) being at right angles to tourmaline (d), of 

 course produces an increase to the polarizing effect. 



The crystal {b) being at right angles to the tourmaline (c), 

 also polarizes and increases the depth of darkness. 



And at the centre (e) we have the combined influence of the 

 two tourmalines and the two crystals also ; we consequently have 

 the maximum polarizing effect which it is possible to produce 

 with this combination. 



In the second place, we will rotate the object through an arc 

 of 45° whilst the tourmalines remain stationary. The crystals 

 are now in the position most favourable for exhibiting depolari- 

 zation : they compel the light to pass through, and at the same 

 time communicate colour to the beam, unless their thickness be 

 too great, when of course white light will be transmitted. (Fig. 9, 

 e, /.) Similar results follow in the examination of hexagonal 

 plates. 



(F.) "We will now proceed to examine these crystals by means 

 of a single tourmaline and the selenite stage. 



This arrangement consists in placing a tourmaline in the 



