The Structure of Coloured Bodies. 199 



its central position, but the lateral spectral images become 

 individually broader and more distantly separated from each 

 other. If the grating be rotated in azimuth the plane of 

 the spectral images also rotates so as always to maintain a 

 direction at right angles to the direction of the lines. Apart 

 from the distinctive appearance of this phenomenon we 

 learn from it that light transmitted by a structure composed 

 of fine lines gives rise to colour — diffraction colours, but 

 the light m the axis of the illuminating beam is colourless. 



If now, adopting precisely the same arrangement of the 

 microscope as in the previous instance, I replace the series of 

 lines by a film of some material sufficiently thin to give rise 

 to the so-called colour of thin plates — say a film of mica — 

 and, using the same objective (lin.), I focus for the small 

 hole in the diaphragm with the film in such a position that 

 the light from this opening on its way to the objective 

 passes through such film, on viewing such opening in 

 the ordinary way through the eye-piece it is seen as a single 

 central faifitly-coloured image. On rotating the film in 

 altitude, it is seen to change in colour. Rotating in azimuth 

 no alteration in appearance is perceptible. From this we 

 learn that the light transmitted by an iridescent thin plate 

 is only in the axis, or parallel to the axis, of the illuminating 

 beam and is coloured. The colour is confined to the direction 

 of the illuminating beam. 



From these two results we learn that in the case of colour- 

 producing structures composed of fine lines, thus examined 

 by transmitted light, the resulting colour is absent in the 

 axis of the illuminating beam, whilst in the case of a colour- 

 producing structure of thin plates, the colour is confined to 

 the direction of the illuminating beam. 



Such is the result of examination by transmitted light, 

 available therefore in the case of transparent structures 

 only. Most of the objects under consideration, however, 

 are opaque bodies, and therefore only admit of examination 



