DARK GROUND ILLUMINATION 87 



front lens of the objective being forced through the 

 cover-glass, and not only spoiling the specimen, but also 

 contaminating the objective) ; but, on the contrary, 

 withdraw his eye, rack the tube up, and commence 

 again from step 2. 



Dark Ground Illumination. 



1 . Set up the microscope stand in the vertical position 

 and insert the highest eyepiece available. 



2. Remove the nosepiece from the microscope tube 

 and fit the inch objective in place. 



3 . Remove the substage condenser and replace it by 

 the dark ground condenser. 



4. Fit up the source of illumination some 30-50 cm. 

 distant from the microscope. (This should be the Lili- 

 put Arc Lamp (Leitz) , Nernst Lamp or incandescent 

 gas lamp ; if either of the two latter are employed, a bull's 

 eye condenser to produce parallel rays must be inter- 

 posed between light and microscope) ; and adjust illumi- 

 nant and microscope so that the substage plane mirror is 

 completely filled with light. 



5. Focus the two concentric rings engraved upon the 

 upper surface of the condenser and centre them accu- 

 rately by means of the centring screws. 



6. Prepare a "fresh" specimen (see pages 74-76) of the 

 material it is desired to observe, using selected, new, 

 3 by i glass slips of less than i mm. thickness, and No. 

 i cover-glasses (0.17 mm. thick), which should be 

 cleaned with a piece of soft washleather and not with the 

 emery paper, as scratches on the glass produce haziness 

 in the preparation). 



7. Deposit a large drop of immersion oil (or pure 

 water) on the upper surface of the condenser and rack it 

 down a few millimetres. 



8. Adjust the fresh preparation on the microscope 

 stage and fasten it in position with the stage clips. 



9. Rack up the condenser until the immersion 



