∞ generated and posted on 2016.01.14 ∞
Highest magnification objective lens in a compound microscope that is not be used with immersion oil.
"High" refers to its magnification while "dry" refers to its lack of immersion in oil. In fact, no objective lens except for the oil immersion lens should be used with oil, care should be taken to avoid getting oil on the high and dry lens, and any oil that accidentally gets on this lens should be quickly cleaned off. The high and dry lens typically is 40× magnification, though can be higher.
The role of the high and dry lens is one of provided greater breadth of field, more light, and more clarity in the absence oil. This provides a convenient means of identifying areas of interest on a slide as well as a fine tuning of focusing before turning to oil immersion. Also, for sufficiently large objects, it allows viewing of a greater amount of it, that is, maximum magnification is not always either necessary or desired and the high and dry lens additionally provides ease of use since oil is not required to achieved reasonably levels of sharpness.