VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 85 



by referring to the diagram, Fig. 39. An ordinary disk (or prism) 

 illuminator I is attached to the tube T of the microscope. Into 

 the lower opening is screwed an aluminum adapter A which serves 

 to hold in position the supporting bar B. The objective O is 

 screwed into the lower end of A. The bar B carries a vertical 

 sleeve J, fitted with a thumb-screw and serving to hold in place 



FIG. 39. Tassin Vertical Illuminator. 



the remaining parts of the illuminator. The sleeve D carries a 

 Ramsden eyepiece, securely held in position by the screw K. 

 This eyepiece acts as a condensing lens. The correct position 

 of the lenses to obtain a spot of bright light of the requisite 

 diameter upon the reflecting surfaces is secured by sliding the 

 entire ocular in the -sleeve or by sliding the lens C or both. The 

 clamping joint E permits tilting the condenser so as to obtain 

 the correct angle of incidence upon the disk or prism. To ex- 

 clude all other light from the illuminator, a screen S is attached 



