38 The Microscope 



3. Remove the slide, rack up the tube, and place a drop of oil on the 

 top lens of the condenser. 



4. Refocus the slide with the X40 objective and then refocus the sub- 

 stage condenser. Rack up the tube and make sure that the whole surface 

 of the top lens of the condenser is joined to the slide with oil. It some- 

 times happens that there is too great a gap between the condenser and 

 the slide. Remove the slide, lay a couple of coverslips on the oil on the 

 condenser, add oil to the top of the coverslips, and replace the slide. This 

 compensates, in effect, for having used too thin a slide. There is no cure 

 for too thick a slide. 



5. Place a drop of oil on the slide. Swing the N.A. 1.4 lens into posi- 

 tion, lower it into contact with the oil, and then focus it. 



6. It will usually be necessary, in practice, to shut down the substage 

 iris slightly— to, say, N.A. 1.3— to reduce glare. 



It may be necessary, after examining one field with this lens, to change 

 slides and search the new slide with a X 10 lens to find the required field. 

 The image of the field iris will of course be minute and that is why the 

 supplementary lens shown in Fig. 38 is provided. Swing or slide it into 

 position and the whole field of a XlO lens will be illuminated. A similar 

 lens is available for an Ortho-Illuminator. 



Most N.A. 1.4 condensers will adequately fill the field of a X20 ob- 

 jective. If much critical work is to be done with a XlO objective it is 

 best to have an N.A. 1.0 Abbe condenser, which can be exchanged for 

 the N.A. 1.4. For occasional critical work, the top lens of the N.A. 1.4 

 condenser may be removed. The last operation to be described is center- 

 ing the stage. 



Centering the Mechanical Stage 



This is usually only necessary when the microscope is to be used for 

 photography. 



1. Set up the X 10 objective. 



2. Place a slide carrying crossed hairs on the stage and center it with 

 the rectangular rack movement. 



3. Unlock the rotating movement (Fig. 47). This extremely important 

 step is often neglected, with consequent damage to the instrument. 



4. Rotate the stage. The cross will describe the arc of a circle. Estimate 

 the position of the center of this circle and try to bring this center into 

 the center of the field with the stage centering screws (Fig. 46). 



5. Use the rectangular rack movement to bring the crossed hairs back 

 into the center of the field. Again rotate the stage. This time the crossed 

 hairs will describe the arc of a much smaller circle. 



6. Repeat steps 4 and 5 until the cross rotates on its own center. 



