PRACTICAL EXERCISES WITH THE MICROSCOPE 267 



46. Measuring the Aperture of an Oil -immersion Objec- 

 tive. — Put a round drop of Canada balsam on a cover-glass 

 over an object (such as the lines of the Abbe test plate). 

 Wait till it has dried enough to form a toughish surface film. 

 Then focus down a 60 oil-immersion objective of 1.3 aper- 

 ture with the front lens clean and dry, pressing in the drop 

 of balsam. If the film does not break, there is a thin layer 

 of air between the front lens and the balsam drop, and the 

 corrected immersion condenser cannot give an aperture 

 over 1.0 (Abbe). Measure this condenser circle on the 

 back of the objective by a low objective screwed into the 

 drawtube, and an eyepiece micrometer scale. Then clean 

 the slide and immerse the objective properly in thickened 

 cedar oil, using a full condenser cone with a large source, so 

 as to fill the back of the objective. Measure the diameter 

 of this circle. Calculate the aperture from the measure- 

 ment of the circle with aperture 1.0, the apertures being 

 proportional to the diameters. (Zeiss, Abbe.) 



47. Marking Apertures of Condenser. — Fix a blank paper 

 scale for the iris handle of the condenser to move by. 

 Correct and immerse condenser. Take the objective used 

 in 46. Use an additional low objective in the drawtube 

 and a micrometer eyepiece. Find the place on the scale for 

 1.0 aperture by opening the iris till the diameter of the 

 condenser circle on the back of the objective is the amount 

 found in 46. From this, calculate the diameters of the 

 condenser circle for 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 aperture. 

 Set the circles at these diameters, and mark corresponding 

 positions of iris handle on scale. 



48. Comparison of Dry and Water-immersion Objec- 

 tives. — Take a dry objective 40 of 0.85 aperture with a 

 correction collar, and also a water-immersion objective 70, 

 also with correction collar. Adjust them for cover thick- 

 ness, and focus both on an object in water. Note that the 

 water-immersion objective gives perfect images at any 

 depth, while the dry objective has to have its correction 

 collar changed for different depths of the object under 

 water. 



