Using+an+eyepiece+graticule

Measurement of specimens seen with a microscope (using an eyepiece graticule)
All microscopic measurements are in micrometres (μm). These used to be called microns, so if you see that term anywhere, it is the same size as a micrometre.


 * There are 1000 μm in 1mm
 * i.e. 1 μm = 0.001mm

An eyepiece graticule (sometimes rather confusingly known as an eyepiece micrometer) you will be able to measure any specimens that you view. The eyepiece graticule has a 10mm distance marked out in divisions; 10 large divisions, each of which is sub divided into ten smaller divisions.

When you look at a specimen you should see the eyepiece graticule/micrometer superimposed over the magnified image of the specimen. Some people find it hard to see the two things at once; if they focus on one, they cannot see the other. This comes with practice. The key points to understand how a graticule works are summarised below


 * When you look at a specimen at low power magnification, with a x10 eyepiece lens and x4 objective lens, the image is magnified 40 times (x10 x4). Try looking at how many divisions 1mm on a ruler covers, it should be 40.
 * When you look at the same specimen at higher magnification, say with the x10 eyepiece lens and a x10 objective lens, the image is now magnified 100 times. Look at the ruler scale again 1mm should now cover 100 small divisions.
 * We want to find the actual size of the specimen. As it has been observed under a higher magnification, it has not actually become larger.
 * Therefore the size of the divisions of the eyepiece graticule is arbitrary.


 * In your notes**
 * Make a heading __Sizes of the graticule divisions at different magnifications__.
 * Copy and complete the table below:


 * Magnification by eyepiece lens || Magnification by objective lens || Total magnification || Value of 1 small eyepiece division (μm) ||
 * x10 || x4 || x40 || 25 (=100 / 4) ||
 * x10 || x10 || x100 ||  ||
 * x10 || x40 || x400 ||  ||
 * x10 || x100 || x1000 ||  ||


 * How to work out the values**

There are 100 small divisions on the eyepiece graticule and the total length of the scale is 10mm. If 1mm is 1000μm, then unmagnified each of the small divisions on the eyepiece graticule is 10,000μm (size of graticule) / 100 (# divisions) = __100 (μm/division)__

__Use the formula:__


 * Value** of 1 small eyepiece division (μm) = **actual** size of 1 small eyepiece division / **magnification** (of the objective lens)

eg. Value of 1 small eyepiece division (μm) at x40 = 100 / 4

n.b. The trick is to remember that the graticule, because it is on the eyepiece lens, is only being magnified by the objective lens.