APPENDIX 



225 



Fl( , 72 



the ray transmitted by the first will be extinguished by the second. Let 



PP represent the principal plane of the interposed doubly -refractive plate. 



The extraordinary ray transmitted by NjNj vibrates in the plane NjN,, and 



falls obliquely on the plate PP ; it is by this plate itself split into two rays, 



an ordinary and an extraordinary one, at right angles to one another, one 



vibrating in the plane PP, the other in the plane P 1 ? 1 . These two rays 



meet the second nicol, which can only transmit vibrations in the plane N 2 N 2 . 



The vibrations in PP can be resolved 



into a vibration in NjNj and a 



vibration in N.-,N a ; the former is 



extinguished, the latter transmitted. 



Similarly the vibration in P 1 ? 1 can 



be resolved into two sub-rays in 



N,N! and N 2 N 2 respectively, the 



latter only being transmitted. The -^ ^ 



illumination is thus due to two 2 



sub-rays, one of the vibrations in 



PP, the other of those in P*P l which 



have been made to vibrate in N 2 N 2 . 



Now, although these two sub- 

 rays vibrate in the same plane, 

 they are of different velocities ; 

 hence the phases of the vibrations 

 do not coincide, and thus the 



phenomena of interference are obtained. If we have two sets of vibrations 

 fused, the crest of one wave may coincide with the crest of the other, 

 in this case the wave will be higher ; or the crest of one may coincide 

 with the hollow of the other, that is, the undulation would be extinguished ; 

 in ether intermediate cases, the movement would be interfered with, either 

 helped or hindered, more or less. Interference in the case of many kinds 

 of doubly-refracting substances (Iceland spar is in this an exception) shows 

 itself in the extinction of certain rays of the white light, and the light seen 

 through the second nicol is white light minus the extinguished rays ; those 

 extinguished and those transmitted will together form white light, and are 

 thus complementary. Moreover, the rays extinguished in one position of 

 the plate will be transmitted in one at right angles and vice versa ; thus a 

 crystal showing these phenomena of pleochromatism, as it is termed, will 

 transmit one colour in one position, and the complementary colour in a 

 position at right angles to the first ; blue and yellow, and red and green, are 

 the pairs of colours most frequently seen in this way. 



Rotation of the Plane of Polarisation. Certain crystals such as those of 

 quartz, and certain fluids such as the essence of turpentine, aniseed, &c., 

 and solutions of certain substances like sugar and albumin, have the power 

 of rotating the plane of polarised light to the right or left. The polarisation 

 of light that is produced by a quartz crystal is different from that produced 

 by a rhombohedron of Iceland spar. The light that passes through the 

 latter is plane polarised ; the light that passes through the former (quartz) 

 is circularly polarised, i.e. the two sub -rays are made up of vibrations 



