154 Mr. Alex. Hodgkinson on 



is examined by light incident, and therefore reflected, at 

 different angles. Thus, different observers see the same 

 iridescent object of a different colour, when they view it 

 illuminated by light at a different angle of incidence. If, 

 however, the object is seen by all at the same angle of the 

 incident light it will present the same colour, and this is, in 

 fact, what the method I propose ensures, i.e., that iridescent 

 objects shall always be seen by light at one and the same 

 angle of incidence. The angle I select is one of 90 , so 

 that the incidence and reflection are normal or perpendicular 

 to the reflecting surface. By selecting this angle all trouble 

 of measuring angles is avoided, since we know that the 

 incidence is perpendicular when it coincides with reflection. 

 Now, the reflected light may be made to coincide with the 

 incident light by reflecting it on to the object by means of a 

 mirror, and so adjusting the object that the light reflected 

 from it passes to the eye through a perforation in the 

 mirror. When examined in this way iridescent objects are 

 marvellously altered in appearance, their changing colours 

 are replaced by one fixed tint, visible only in one position, 

 a fact which serves at once to distinguish them from bodies 

 coloured by absorption, which remain coloured whatever 

 the relation to the incident light. Such methods of 

 examining bodies scarcely takes more time than by the eye 

 alone. The mirror may be attached to a spectacle frame so 

 as to leave both hands free, such as the one I show, or may 

 be a simple hand mirror. For objects too small to be seen 

 by the unaided eye, I have so arranged the microscope that 

 light is made to pass down the tube of the instrument 

 through the object glass on to the objects, and by a special 

 arrangement, so adjusted the position of the object that 

 the light is reflected back again through the instrument to 

 the eye. The method is thus available for macroscopic as 

 well as microscopic objects. 



To illustrate the practical value of this plan of examina- 



