Coloured threads Slide, w.m
This whole mount of 3 different colored threads is perfect when teaching your students how to focus the compound microscope. Students use the fine focus to determine in which order the threads overlap. With the intersection of the threads in the field of view, they learn that only 1 thread at a time can be in focus, illustrating the concept of depth of field.
The red thread lies across the blue thread, which in turn lies across the yellow thread.