130 POLARISED LIGHT AND ITS APPLICATIONS 



Adjust the polariser and analyser of the microscope so as to 

 give a dark ground, and place a slide of selenite on the stage. 

 On rotating the selenite into a certain position the beautiful colours 

 will all disappear, leaving a black ground. If the selenite is 

 rotated through a right angle from this position, the colours will 

 again disappear (to perform the experiment properly, the micro- 

 scope should be furnished with a rotating stage). In either 

 position, one of the two optic axes of the selenite coincides 

 with the direction in which the light is polarised by the polariser. 

 On turning the polariser through a right angle, a white field of 

 view will be obtained just as if there were no selenite. On the 

 other hand, the brightest colours will be obtained when the selenite 

 is rotated through 45*^ (half a right angle) from either of the 

 aforementioned positions. 



Thus, to obtai?i the best effects, the light should be polai'ised in a 

 direction inclined at Of^^"^ to {i.e., half-way bettueen) the directiofis of 

 the optic axes of the section. 



Pendulum Experiment.— To completely explain on the wave 

 theory the changes which take place in a beam of polarised light 

 as it travels through the section is very difficult. If we rely 

 exclusively on theory, we cannot help finding ourselves face to 

 face with mathematics, or something very like it. The following 

 very pretty pendulum experiment, however, affords an excellent 

 illustration of the phenomena, and as it requires no special appa- 

 ratus I hope all my readers will perform it for themselves :— 



Take two pieces of string (PI. VII., Fig. 3), knot them 

 together at C to form a Y? ^iid attach the shorter ends A, B 

 to a horizontal bar (such as that of a towel-horse) a few inches 

 apart ; or (what amounts to the same thing) tie a loop in the 

 string and pass it over a narrow plank whose breadth is AB, so 

 that it should hang in a Y as before. To the longer end (which 

 should be a foot to 18 inches in length), attach any convenient 

 weight P — say, \ lb. 



If the weight be pulled out of the vertical to a point E in the 

 plane of the strings (as in Fig. 3), it will swing to and fro along 

 EE after the manner of a pendulum attached to a fixed support 

 at C, and the upper arms, AC, BC, will remain at rest. Next, 



