2.1- Measuring objects seen with a light microscope Flashcards
What is an eyepiece graticule?
A measuring device. It’s placed in the eyepiece of a microscope and acts as a ruler when you view an object under the microscope.
What is a stage graticule?
A precise measuring device. It is a small scale that is placed on a microscope stage and used to calibrate the value of eyepiece divisions at different magnification.
How is a graticule used?
- A microscope eyepiece can be fitted with a graticule.
- This graticule is transparent with a small ruler etched on it.
- As the specimen is viewed , the eyepiece graticule scale is superimposed on it and the dimensions of the specimens can be measured (just as you can measure a large object by placing a ruler against it) in eyepiece units.
What is the scale of the eyepiece graticule?
The scale of the eyepiece graticule is arbitrary- it represents different lengths at different magnifications. The image of the specimen looks bigger at higher magnifications, but the actual specimen has not increased in size.
Why does the eyepiece scale have to be calibrated (its value worked out)?
The eyepiece has to be calibrated for each different objective lens.
How do you use a stage graticule to calibrate the eyepiece graticule?
1) Insert an eyepiece graticule into the 10 eyepiece of your microscope. This ruler has a total of 100 division.
2) Place a stage graticule on the microscope and bring into focus using the low-power (4) objective. Total magnification is now *40.
3) Align the eyepiece graticule and the stage graticule. Check the value of one eyepiece division at this magnification on the microscope.
4) In the example (in book pg 33.) , the stage graticule (which is 1mm) corresponds to 40 eyepiece divisions.
5) Therefore each eyepiece division =1000/40um=25um
6) Now use the *10 objective lens on microscope (magnification is *100) and focus on the stage graticule.
7) Align them both
8) In the example, 100 eyepiece divisions now corresponds with 1mm/ 1000um.
9) Therefore one eyepiece division=1000/100um=10um
How long is the stage graticule?
The ruler is 1mm long and divided into 100 divisions. Each division is 0.01mm or 10um (micrometres)
Describe the investigation- observing and measuring starch grains (amyloplasts) in potato tuber cells:
Wear eye protection
1) Using a sharp knife, gently scrap a little material from the surface of a peeled raw potato and place it on to a microscope slide. You need a very thin layer on the slide. In that material will be potato tuber cells.
2) Place 2 drops of iodide/ potassium iodide (Kl) onto them and carefully add a coverslip.
3) Examine this slide under the microscope. Use low power first and then use high power magnification. The amyloplasts stained with iodine solution will appear violet in colour.
4) Measure the length and width of three amyloplasts.
