Measuring cells

• What is an eyepiece graticule?


  A glass disc that's placed in the eyepiece of a microscope.

  A scale is etched on the glass disc.

  This scale is typically 10mm long and is divided into 100 subdivisions

  The scale is visible when looking down the eyepiece
• Can we use an eyepiece graticule to measure objects?


  No. each objective lens on a light microscope magnifies to a different degree.
• The graticule remains constant no matter what mag the cells are viewed at, how do we overcome this problem?


  The graticule must be calibrated to the objective lens.
• What piece of equipment do we need for calibration?


  A stage micrometre- a microscope slide with a 2mm scale and 0.01mm sub-divisions (usually) etched onto it.
• How do you calibrate an eyepiece graticule?


  When the graticule and stage micrometre and aligned you can calculate the length of the divisions of the graticule.

  If 10 units on the micrometre scale equals 40 units on the graticule then 1 equals 4.

  If each micrometre unit equal 10micrometres, each graticule unit equals 10 divided by 4 = 2.5micrometres.
• Instructions for the practical in lesson.

  Place graticule in an eyepiece lens.

  place micrometre on the stage.

  Line them up.

  Use the graticule to measure the width of the specimen.

  Draw your specimen and the graticule. Include the calculated actual size of the specimen.

  Increase the mag of the objective lens.

  Repeat steps.