248 USE OF THE MICEOSCOPE. 



blue of the ordinary and extraordinary systems respectively 

 meeting together, with a difference of an odd number of 

 half undulations, and thus neutralizing each other by inter- 

 ferences. 



" N, red light, the result of the coincidence of the -waves for 

 red light, and the neutraUzation by interferences of those for 

 yeUow and blue respectively. 



" H, in the lower part of fig. 157, represents dipolarized 

 light. 



" I, the analyzer turned one quarter of a circle, its axis 

 being at right angles to that of I, in the upper part of the 

 same figure. 



" K, the waves E° and E," for red light of the ordinary and 

 the extraordinary systems meeting together with a difference of 

 an odd number of half undulations, and thus neutralizing each 

 other by interference. 



" L, M, the waves Y" Y' and B" B" for yellow and blue of 

 the two systems severally meeting together in the same state 

 of vibration, occasioned by the difference of an even number 

 of half undulations, and forming by their coincidences waves 

 of double intensity for yeUow and blue Hght. 



" N, green light, the result of the coincidences of the waves 

 for yellow and blue light respectively, and the neutralization 

 by interference of those for red. 



" By using a plate of selenite of uniform thickness, the 

 colour win be uniform, whereas a plate of different thicknesses 

 will produce different colours, following the same order as 

 those of Newton's rings ; red being produced by the thickest, 

 violet by the thinnest, and intermediate colours by intermediate 

 thicknesses of the plates of selenite." 



By substituting Nicol's prisms for the two plates of tour- 

 mahne, and by the addition of an object-glass and eye-piece, the 

 diagrams would then represent the passage of the light through 

 an achromatic compound microscope. If, instead of the sele- 

 nite, other crystals, such as Iceland spar, quartz, nitre, &c., 

 cut in the manner described at page 240, be placed between 

 the tourmahnes, coloured rings will be produced, which, in 

 some cases, may be intersected either by a black or white 

 cross, according as the light is stopped or transmitted. All 



