34 THE USE OF THE MICROSCOPE 



Water-immersion objectives are best used on objects 

 in water. If used on objects in balsam, errors are intro- 

 duced, increasing with the thickness of the layer of balsam, 

 though these errors can be compensated for by altering 

 the correction collar for every change in the thickness 

 of the layer. If used on objects in air, the aperture is cut 

 down to 1.0, at most. These objectives require covers 

 near 0.17 millimeter thick for the best images. Water- 

 immersion objectives are convenient, and almost essential 

 when many mounts are made in watery fluids. They give 

 better images than dry objectives on objects in water or 

 balsam; and often give better images than oil-immersion 

 objectives on objects in water; especially with covers 0.17 

 miUimeter thick. 



The high oil-immersion objectives are to be used 

 primarily on objects mounted in balsam or immersion oil. 

 They can be used without a cover-glass if the tube length 

 is slightly increased (by about 25 miUimeters for the 100 

 fluorite objective) to correct for the lessened refraction, 

 or if the objectives are specially corrected for use without a 

 cover-glass, as are the best of those used in metallurgy. 

 Oil-immersion objectives give poorer images of objects 

 mounted in air, the aperture being cut down to a limit of 

 1.0. But if objects are in optical contact with the under 

 side of the cover-glass, they may be well seen with the 

 oil-immersion objective, even though mounted in air. 

 Such objects are diatoms fused to the cover, or connected 

 with it by some transparent high-refractive substance, 

 such as hyrax; also bacteria or spirochsetes smeared on the 

 cover, stained, and dried. High oil-immersion objectives 

 can only be well used on objects in a w^atery liquid which 

 are less than about 10 microns from the under side of the 

 cover. This condition must usually be compensated for 

 by appropriately increasing the tube length. When used 

 in this way, the aperture will be cut down by total reflection 

 to an upper limit of usually 1.33 (or sHghtly more, in case 

 of optical contact with the cover) . 



