Theiris diaphragm control lever should be set at at around 0.9 for 1000X oil immersion microscopy. For lower magnifications, reduce the light level of the iris diaphragm. 

Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 3.0 Unported License