VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 8 1 



that the examination with the vertical illuminator be supple- 

 mented by very oblique illumination and that due account be 

 taken of the directions of shadows with respect to the radiant, 

 remembering of course that in the image seen in the microscope 

 directions are completely reversed. 



Polarized Light with Vertical Illuminators. A further aid 

 in differentiating between the phases present in a given specimen 

 is afforded by employing polarized rays for illumination or 

 analyzing the light rays reflected from the object. The light 

 rays reflected from the polished surfaces of sections of aniso- 

 tropic crystals are polarized, as has been already stated, while 

 the rays reflected from isotropic crystal sections are not polarized. 

 It is evident that if we pick out a given phase and employ a 

 magnification, such that an area of this phase alone fills the field, 

 we can, by studying the nature of the light reflected therefrom, 

 often obtain information of the greatest value as to the nature 

 of the composition of the specimen being studied. In many 

 instances it is not even essential to confine one's attention to a 

 selected small area but we may use low powers which will include 

 several phases in the image. 



FIG. 36. Nachet Vertical Illuminator. 



Nachet Vertical Illuminator. 1 This instrument, Fig. 36, 

 consists of a collimator tube C attached to a cell F, which in turn 

 slips into the threaded adapter A and is held in place by the 

 thumb-screw B. The adapter A carries at its upper end a male 

 screw thread of standard pitch, serving to fasten the device into 

 the end of the tube T of the microscope, while F is tapped with 



1 Manufactured by A. Nachet et Fils, Paris, France. 



