356 OPTTCAL PROJECTION 



in thickness is doubled, while in that rotated 180 it is 

 counteracted, and a flat tint is produced equal to the sum of 

 the thickest band and the thinnest ; and if then one wedge 

 be turned over, in one position the whole appears black. 



The most important and useful preparation is a long 

 wedge built up of twenty-four films each ^ wave thick. 1 

 This gives the first three orders of Newton in wave grada- 

 tions. The twelfth band being 1| wave thick, if another 

 plate of mica of uniform 1^ wave thickness be super- 

 posed, with its axes crossed, the twelfth band will appear 

 black on the dark field, and on each side of it will appear, 

 apparently, a wedge resembling the original one, starting from 

 its thin end. Finally, if a slit be inserted in the stage with 

 the wedge, crossing all its films, spectrum analysis through 

 the prism will show exactly what waves are destroyed by 

 interference in each thickness. By having mica or selenite 

 plates of one or more waves, to superpose (the same way) on 

 the Fox wedge, these interference bands may be demonstrated 

 up to any desired order of colours. 



Various and beautiful phenomena may further be shown 

 on the screen by rotating an uniformly thick mica or selenite 

 plate (commonly called an ' even ' film or superposition film) 

 over some other preparation, because in one position the 

 resultant will equal the sum of the two films, and in another 



1 A film by which one of the rays is retarded ^ wave more than the other, 

 is called an ^\ or ^ wave film, and so we have quarter or half- wave, one wave, 

 two waves, &c. This can however only be precisely the case for any one 

 spectral colour at a time, and the adjustment is usually made for the yellow 

 ray, as being brightest and of medium wave-length. Taking then a whole- 

 wave film, or the eighth band in Fox's wedge, one yellow ray is retarded 

 exactly a wave, and the dark field would therefore in monochromatic light 

 give this band perfectly black. But of the red and blue, which are greater and 

 less in wave-length, a very little must be transmitted in white light, and these 

 residuals make the band (or a whole-wave plate) of a dull plum-colour, known 

 as the ' tint of passage,' or transition-tint, between the first and second of 

 Newton's orders. All other thicknesses have similar residuals, which are the 

 same in all interference colours ; these films, and especially the Fox wedge, 

 being the most perfect means of demonstrating them analytically in detail. 



