- Ocular micrometer
- Stage micrometer
- Millimeter ruler
- Prepared slide
- Place a stage micrometer on the microscope stage, and using the lowest
magnification (4X), focus on the grid of the stage micrometer.
- Rotate the ocular micrometer by turning the appropriate eyepiece. Move the
stage until you superimpose the lines of the ocular micrometer upon those
of the stage micrometer. With the lines of the 2 micrometers coinciding at
one end of the field, count the spaces of each micrometer to a point at
which the lines of the micrometers coincide again.
- Since each division of the stage micrometer measures 10 micrometers, and
since you know how many ocular divisions are equivalent to 1 stage
division, you can now calculate the number of micrometers in each space
of the ocular scale.
- Repeat for 10X and 40X, and 100X. Record your calculations below:
Using the stage micrometer, determine the smallest length (in microns) that
can be resolved with each objective. This is the measured limit of resolution for
Compare this value to the theoretical limit of resolution, calculated on the
basis of the numerical aperture of the lens and a wavelength of 450 nm (blue
Using the calculated values for your ocular micrometer, determine the
dimensions of the letter “e” found on your microscope slide. Use a millimeter
ruler to measure the letter “e” directly and compare it with the calculated
values obtained through the microscope.
To measure an object seen in a microscope, an ocular micrometer serves as a
scale or rule. This is simply a disc of glass upon which equally spaced divisions
are etched. The rule may be divided into 50 subdivisions, or more rarely, 100
subdivisions. To use the ocular micrometer, calibrate it against a fixed and
known ruler, the stage micrometer. Stage micrometers also come in varying
lengths, but most are 2-mm long and subdivided into 0.01-mm (10-micrometer)
lengths. Each objective will need to be calibrated independently. To use, simply
superimpose the ocular micrometer onto the stage micrometer and note the
relationship of the length of the ocular to the stage micrometer. Note that at
different magnifications, the stage micrometer changes, but the ocular micrometer
is fixed in dimension. In reality, the stage micrometer is also fixed, and what
is changing is the power of the magnification of the objective.