Oil immersion techniques, first suggested by Robert Hooke in 1678 and first used in 1840, allow for greater magnification while still retaining full optical resolution. A microscope lens creates an image by reconstructing the light scattered by the specimen. The best image is produced when all the light scattered by diffraction is collected. The amount of light that can be collected is determined by the opening angle of the lens and its refractive index. The numerical aperture of a lens is a measure of how much light it can collect. The greater the refractive index of the lenses material, the greater the numerical aperture and therefore the more light that can be collected. When light from the specimen has to pass through air before it travels through the lens, the numerical aperture is limited by the relatively low refractive index of air (1). The oil immersion technique involves placing a drop of oil with the same refractive index of glass between the coverslip of the microscope slide and the objective lens. This reduces the three mediums (slide, air and objective lens) into a single medium with one refractive index. Because oil has a higher refractive index than air, the microscope is able to collect more light and resolve smaller details.
When studying very small specimens, a “dry lens” which does not use oil immersion techniques begins to experience heavy blurring at magnifications of 400x. An oil lens, however, can exceed a magnification 1000x while still preserving good resolution. This model of microscope was used in the UQ Biology labs for around twenty years because the oil immersion technique allowed for close study of microbiology.
The Munitions Supply Laboratories (MSL) was formed in 1922 in Melbourne with the intention to standardize the technical procedures used in Victoria’s defense factories, and also to test their products. Their supervision and testing ensured that the factories’ products were of high quality which was vital to Australia’s defense during World War II. The physics department of MSL was responsible for the design and production of optical devices like this microscope, as well as gun sights and sniper scopes.
This item is part of the UQ Physics Museum Microscopes Tour JDB, SN
