200 Dr. a. Hodgkinson on 



by reflected light. To make our investigation complete, 

 therefore, it behoves us to examine the manifestations of 

 these two colour-producing structures, fine lines and thin 

 plates, by reflected light. 



Using the same objects, a series of engraved lines, and a 

 film of mica, I will first consider the fine lines. 



Placing this object on the stage of a binocular micros- 

 cope and employing a i-inch objective, the following 

 method of illumination is employed : — Removing one of the 

 eye-pieces, I substitute a mirror so arranged that a beam of 

 light may be reflected down the tube and through the 

 objective on to the object beneath. If now the plane of 

 reflection of the object is normal to the direction of this 

 beam, the light is reflected up the other tube, forming an 

 image of the object which is obscured in the usual way. 

 Such image i?i the present instance is seen to be colourless. 

 Rotated in altitude at any azimuth, the image disappears. 

 From this we learn that a colour-producing structure of fine 

 lines reflects colourless light at an angle equal to that of 

 incidence. The above arrangement ensures incidence and 

 reflection being equal, since they are identical, being both 

 normal to the reflecting plane. 



Replacing the fine lines by an iridescent film of mica, 

 mounted on black velvet so as to avoid the reflection of 

 adventitious light, and subjecting it to precisely the same 

 method of examination as in the last instance, a striking 

 difference is noticed in the result. Instead of the colourless 

 image of the object as observed in the previous instance, 

 the image of the film appears as an intensely brilliant object 

 tinged with hues which, though they may be equalled, are 

 certainly unsurpassed, even by the interference colours of 

 polarised light. If by tilting the stage the film is made to 

 rotate in altitude at any azimuth, the colours immediately 

 disappear. From this we learn that a colour-producing 

 structure of thin plates reflects a coloured light only at an 



