What Power Should Be Used on a Compound Microscope?
- A condition that's true for optical instruments in general is that higher magnification is not necessarily better. This holds for telescopes, binoculars and microscopes. Related to this is a consideration that's not so much a factor in very high-quality instruments with refined optics: poor image quality at high magnifications. With mid-level to low-quality instruments, higher magnification can result in loss of resolution and other image degradation because of optical defects. These become exaggerated at higher magnifications. In this case, choose low to medium powers because these will afford the best views.
- A 1000X lens is a common feature of quality compound light microscopes. (A power of 1000X results from the combination of a 100X objective lens and a 10X eyepiece lens.) This is a very high magnification for an optical light microscope. Simply viewing the specimen on a slide with this power, though, in the same way that the other objective lenses are used would result in considerable loss of image quality. This is because light passing into the objective lens then to the observer's eye passes through air between the objective lens and the microscope slide. Air has a different so-called refractive index from the optics in the microscope's lenses. This tends to degrade the image quality. This is not much of a factor with the lower-powered objectives, but with the 1000X it becomes a problem.
- The solution to this is something called oil immersion. The oil immersion lens is designed to be situated above the slide and brought to focus in such a way that there is a relatively small distance between the lens and the slide or coverslip. In this space, instead of allowing air to be in the path of light, you'll turn the objective turret so that it's in between objective lenses. Then you'll put a drop of what's called immersion oil on the slide's coverslip and rotate the turret so that the oil immersion lens comes into contact with the oil. Immersion oil is formulated to have a refractive index very similar to that of the microscope's optics. This technique improves image quality markedly and allows for viewing at what for a light microscope is very high power. So be sure to keep a bottle of immersion oil handy.