Determining the depth of focus using the crossed threads slide

The location in vertical space where your image is focused is called the plane of focus. As you

focus the specimen, you will notice that there is a range of locations where the sample is still in focus.

This is called the depth of focus. The depth of focus depends on the objective lens.

$1.$ Obtain a colored threads slide from the tray at the front counter

$2.$ Under the scanning $(4$x$)$ lens, adjust the course focus and check which way brings the slide closer

to or farther from the lens.

$3.$ Rotate the low $(10$x$)$ power lens into position. Using the stage controls move the slide until the

region where the threads overlap is in the center of the field of view.

$4.$ Using only the fine focus knob, focus up and down. As you do so you will observe different

threads become distinct; when one thread is in focus, the others are blurred. By focusing

continuously up and down through the slide you can perceive the dimension of depth that is not

revealed when you focus at just one point.

$5.$ Rotate the high $(40$x$)$ lens into place. Use the fine focus to focus up and down. Notice how the

different parts of the threads are sharp in some focal planes and fuzzy in others.

$6.$ Determine the order of overlap of the threads on your slide, from top to bottom.

List the order of colored threads from top to bottom as they appear on your slide:

$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$

$7.$ Return the crossed threads slide to the slide tray. Do not put into the dirty slide discard bucket.