Iridescent Colours. 157 



electric light probably almost as good. I frequently employ 

 the limelight, but a good paraffin lamp may be used as a 

 substitute. Ordinary gas is unsuitable. The light should 

 be placed in front of the observer, its direct rays being pre- 

 vented from falling on the objects by means of a book or par- 

 tition of some kind resting on the table, and of such a height 

 that the light can be seen above it. On placing the mirror 

 to the eye the light may be reflected from the mirror on to 

 the object, and the latter manipulated so as to reflect the 

 ray back through the perforation in the mirror to the eye. 

 The incidence is thus known to be normal, and the colour 

 observed is the one to be recorded. 



