Compound microscopes have assisted scientists in the search for objects invisible to the naked eye for more than four hundred years and have greatly influenced our understanding of the world around us. With the advancement of technology, optical microscopy has improved greatly. These improvements include lighting methods, quality of resolution lenses, 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 and the object at the end of the tube appeared to be greatly magnified ( history-of-the-microscope.org). This microscope consisted of two lenses positioned at each end of a compactable tube. As the tube extended, the distance between the lenses increased and the image of an object was magnified. Unlike the modern compound microscope, there were no methods of illumination or the 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 can be smaller than one wavelength of light and therefore will not be visible under the microscope. However, the wavelengths of light in microscopes can be changed so that light can pass through an object and be magnified. Incandescent bulbs with tungsten filaments are generally 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 blurry image of an object due to parallel light rays passing through the center of the paper. .....The slide and the highest power objective lens are lowered until a "bridge" is created between the objective lens and the coverslip. Immersion oil has a refractive index close to the refractive index of glass; allowing minimal refraction of light rays. The quality of resolution can be significantly improved by the use of immersion oil as it increases the numerical aperture of the objective lens (microscope-microscope.org). Oil immersion eliminates color defects and helps stop spherical aberration by producing a partially converging cone of light before entering the objective lens. With an object at the aplanatic point (the focal point and center of the field) of the objective lens, there is no spherical aberration. Oil immersion greatly improves resolution, corrects color defects, and stops spherical aberration (microscope-microscope.org).