260 THE USE OF THE MICROSCOPE 



microscope (that is, when tlie microscope tube has to be 

 raised from the focus to get the mist) means undercorrection 

 and too thin cover-glass; while mist within the focus of 

 the microscope (so that the microscope tube has to be 

 lowered from the focus to get the mist) means overcorrec- 

 tion, and too thick cover-glass. 



Take out one eyepiece of the binocular. With the Abbe 

 test plate in focus, get the back lens of the 40 objective 

 nearly quite filled with light. Put a card in from the front 

 of the microscope just below the condenser so as to cover 

 two-thirds or more of the diameter of the condenser cone as 

 seen on the back of the objective. Then, without altering 

 the focus, look through the other eyepiece at the thin bars 

 of silver film at a point near the center of the field. If the 

 cover is too thin, the upper edge of the line of silver film 

 will be foggy and the lower edge sharp. If the cover is 

 too thick, the upper edge wdll be sharp and the lower foggy. 

 In the writer's experience, this test is not so delicate as 

 the star test. (Siedentopf, Colfes, Spitta.) 



28. Use of Appropriate Immersion Oil. — Take an oil- 

 immersion objective of 1.3 aperture, and see that the front 

 lens is free from dry immersion oil. Focus it carefully, 

 without using oil, on a fine well-stained object, under a 

 standard cover-glass, with a corrected condenser and a 

 fairly large cone. Observe the lack of definition. Now 

 run distilled water between objective and cover, and 

 observe, on refocusing, the improvement in definition; 

 which is, however, still imperfect. Dry the slide and objec- 

 tive with blotting paper and put ordinary thick paraffin 

 oil between them. Note that the definition is better, but 

 still quite bad. Finally, clean off with xylol, and use 

 the proper thickened immersion cedar oil, and note the 

 improvement. Observe that larger condenser cones can 

 be used with the correct immersion fluid. 



29. Use of Correction Collar. — Take a dry or water- 

 immersion objective with a correction collar. Measure 

 the cover-glass on an object, approximately, with the screw 

 cover-glass gage; by measuring first the slide, and then the 



