200 PHOTOMICROGRAPHY 



scope and lamp can be similarly made for this optical 

 bench, but in the illustration Zeiss' triangular bar, carrying 

 cooling trough, Kohler condenser, and mirror, on saddle 

 stands, is used instead, and placed parallel to the micro- 

 scope. A convergent beam of light is projected by 

 condensers and mirror into a nose-piece vertical 

 illuminator, and reflected thence by the cover-glass 

 through the objective on to the prepared surface of the 

 metal. The microscope is the Edinburgh Student's " H," 

 and has not got a focussing stage. The substage is there- 

 fore made use of for carrying and focussing the metal. 

 A tube is screwed in the position usually occupied by the 

 condenser ; it is filled with plasticine and the metal 

 imbedded in this, care being taken that its prepared 

 surface is horizontal by pressing it against a slide held 

 across the microscope stage. This method of holding 

 the metal has the advantage that only one surface of the 

 latter has to be prepared, whereas if it is laid on a stage 

 two surfaces have to be polished truly parallel. The 

 condensers are adjusted to give critical or nearly critical 

 light on the metal surface, which is brought into focus by 

 means of the substage rack and pinion, and finally by 

 the ordinary fine adjustment. The two photographs of 

 Spiegeleisen given on Plate 28 were taken in this way, 

 and the particulars given there will show the exposures, 

 etc., required. The optical bench of this camera is as a 

 rule fixed in line with that of the microscope, and is only 

 moved to the side for such special purposes as metal- 

 lography. 



Another device whereby an ordinary microscope , fitted 

 with vertical illuminator, can be used to photograph an 

 object laid upon its stage is shown in diagram at Fig. 66. 

 A reflecting prism is attached to the nose-piece and 

 directs a parallel beam of light into the illuminator 

 however much the microscope tube is moved during 

 focussing. 



