APPENDIX 



189 



vibrating in the plane PP, the other in the plane PT^ These two rays 



meet the second nicol, which can only transmit Aribrations in the plane 'is.^lH^. 



The vibrations in PP can be resolved 



into a vibration in NjNj and a 



vibration in NjN^ ; the former is 



extinguished, the latter transmitted. 



Similarly the vibration in P'P^ can 



be resolved into two sub-rays in 



NjN, and NoN^ respectively, the 



latter only being transmitted. The 



illumination is thus due to two 



sub-rays, one of the vibrations in 



PP, the other of those in P^P' which 



have been made to vibrate in N.^Nj. 



Now, although these two sub- 

 rays vibrate in the same plane, 

 they are of diflferent velocities ; 

 hence the phases of the vibrations 

 do not coincide, and thus the 



phenomena of interference are obtained. If we have two sets of \abrations 

 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 other 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 hght 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 pleochromatisiyi, as it is termed, wUl 

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

 position at right angles to the first ; blue and j-ellow, and red and green, are 

 the pairs of colours most frequently seen in this way. 



Botation 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 

 which occur not in a plane, but are curved. The two rays are circularly 

 polarised in opposite directions, one describing circles to the left, the other 

 to the right ; these anite on issuing from the quartz plate ; and the net 

 result is a plane polarised ray with the plane rotated to right or left 

 according as the right circularly polarised ray or the left proceeded through 



