A graticule is a measuring device used primarily within scientific instruments, such as microscopes, to accurately quantify the size and dimensions of microscopic specimens. This tool is used in fields like biology and materials science to collect precise dimensional data for cells, bacteria, and other minute structures. Incorporating this measuring scale directly into the optical pathway allows researchers to gather quantitative data, converting the magnified image into a reliable, real-world measurement.
Defining the Graticule and Its Location
A graticule is a physical component, typically a small, transparent glass disk featuring a finely etched, standardized scale that resembles a ruler. This disk is carefully inserted into the eyepiece (ocular lens) of a microscope. The placement within the eyepiece ensures the scale is positioned at the instrument’s primary image plane.
Because the graticule is fixed within the optical system, its scale appears visually superimposed over the specimen when a user looks through the eyepiece. This superimposition allows for direct comparison between the specimen’s size and the divisions on the scale. The scale often spans 10 millimeters and is divided into 100 smaller units.
The Two Essential Types: Ocular and Stage
Accurate measurement in microscopy requires the use of two distinct components that work in tandem: the ocular graticule and the stage graticule. The ocular graticule, sometimes called a reticle, is the scale housed inside the microscope’s eyepiece. The divisions on this scale are arbitrary, meaning they do not inherently represent a standard unit of length like millimeters or micrometers.
The stage graticule, or stage micrometer, is the necessary reference standard for making accurate measurements. This is a specialized glass slide placed directly on the microscope stage, similar to a specimen slide. The stage micrometer has a scale of known, precise measurements etched onto its surface. Typically, this scale features divisions that are exactly 0.01 millimeters (10 micrometers) apart. The stage graticule is used to define the arbitrary units of the ocular graticule.
Practical Application: Calibration for Accurate Measurement
The arbitrary nature of the ocular graticule’s scale means it must be calibrated against the known standard of the stage micrometer before any specimen measurement can be performed. Calibration is required because the apparent size of the ocular divisions changes with the objective lens used, as higher magnification causes the scale to appear larger. The scale must be redefined for every objective lens, ensuring the accuracy of the final measurement.
The initial step in this procedure involves placing the stage micrometer on the microscope stage and bringing its precise scale into focus. The ocular graticule’s scale is simultaneously visible, superimposed over the stage micrometer scale. The user must rotate the eyepiece until the lines of the ocular graticule are parallel to the lines of the stage micrometer.
Next, the two scales are aligned, typically by moving the stage micrometer until a zero line on both scales perfectly coincides. The user then identifies a second point where a line on the ocular graticule aligns exactly with a line on the stage micrometer, finding the greatest number of perfectly overlapping divisions possible. This comparison determines how many ocular divisions correspond to a known distance on the stage micrometer. For example, a comparison might show that 40 ocular divisions span the same distance as four divisions on the stage micrometer.
Since each stage micrometer division is 0.01 millimeters (10 micrometers), the four divisions represent a known length of 40 micrometers. A simple calculation is then performed to determine the calibration factor for that specific magnification: 40 micrometers divided by 40 ocular divisions equals 1 micrometer per ocular division. This calculation is a required step for every objective lens on the microscope. Once the calibration factor is calculated, the stage micrometer is removed and replaced with the actual specimen. The calibrated ocular graticule can then be used to measure the specimen by counting the number of ocular divisions the object spans and multiplying that count by the calculated factor.