234 



PROJECTION WITH SUBSTAGE CONDENSER [Cn. IX 



FIG. 131. FILAR MICROMETER OCULAR. 

 (Cut loaned by the Bausch & Lomb Optical Co.}. 



This filar micrometer ocular is of the Ramsden type and consists of a positive- 

 ocular with a movable hair line and two reference lines at right angles to each 

 other as shown in A. The movable line must be carried over the entire length 

 of the object to be measured by rotating the drum. 



A Field of the filar micrometer showing the movable and the cross lines, 

 and the comb. The teeth serve to measure the total revolutions of the'drum. 



FIG. 132. ILLUMINATING OBJECTS OF VARIOUS SIZES ix MICRO-PROJEC 



TION WITH THE MAIN CoNDEXSER OxLY. 



The object must be put in the cone of light at a point where it will be fully 

 illuminated. 



For high powers it will be at or very near the focus (/). For larger objects 

 and low powers the object is at 2 or j, or even closer to the condenser face. 



Arc Supply The right-angled carbons of the arc lamp. 



L 1 L 2 The first and second elements of the triple condenser. 



Water- Cell The water-cell for absorbing radiant heat. It is in the parallel 

 beam between the first and second elements of the condenser. 



Axis The principal optic axis on which all the parts are centered. 



If only the main condenser is used (fig. 121), the cone of light 

 from the condenser must be sufficient to fill the aperture of the 

 projection objective. This requires that the second clement of the 



