Compound microscopes have helped scientists search for objects invisible to the naked eye for over four hundred years and have greatly influenced our understanding of the world around us. As technology has advanced, optical microscopy has improved significantly. These improvements include illumination methods, lens resolution quality, and the use of oil immersion. The first compound microscope was invented by Zacharias Jansen and his father Hans in 1595. While experimenting with lenses in a tube, Zacharias and his father made an important discovery, where the image of the object at the end of the tube appeared considerably enlarged (history-of-the-microscope.org). This microscope consisted of two lenses positioned at each end of a compactable tube. As the tube expanded, the distance between the lenses increased and the image of an object became larger. Unlike the modern compound microscope, there was no illumination method or use of oil immersion and the quality of the lenses was extremely poor. Illumination Compared to the first compound microscope, modern compound microscopes use various illumination methods to produce high-quality images. Objects may be smaller than one wavelength of light and therefore will not be visible under a microscope. However, the wavelengths of light in microscopes can be changed so that light can pass through an object and be magnified. Tungsten filament incandescent bulbs are typically used in microscopes and emit wavelengths of light. The longest wavelengths are towards the infrared end of the spectrum and the shortest wavelengths are towards the ultraviolet end of the spectrum. Spherical aberration is the blurred image of an object due to parallel light rays passing through the middle of the paper. .....the slide and the highest power objective are lowered until a "bridge" is created between the objective and coverslip. Immersion oil has a refractive index close to that of glass; allowing very little refraction of light rays. The quality of resolution can be significantly improved by the use of immersion oil because it increases the numerical aperture of the objective (microscope-microscope.org). Oil immersion eliminates chromatic defects and helps stop spherical aberration by producing a partially converged cone of light before entering the lens. With an object at the aplanatic point (the focal point and center of field) of the lens, there is no spherical aberration. Oil immersion significantly improves resolution, corrects chromatic defects and stops spherical aberration (microscope-microscope.org).