1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
Objective lenses: One of the most important parts of a compound microscope, as they are the lenses closest to the specimen. A standard microscope has three, four, or five objective lenses that range in power from 4X to 100X. When focusing the microscope, be careful that the objective lens doesnât touch the slide, as it could break the slide and destroy the specimen. Specimen or slide: The specimen is the object being examined. Most specimens are mounted on slides, flat rectangles of thin glass. The specimen is placed on the glass and a cover slip is placed over the specimen. This allows the slide to be easily inserted or removed from the microscope.