something by moving the object under observation to the end of the pointer. Examine your slide, starting with the lowest power objective and progressing to the  highest power objective (do NOT use the oil-immersion objective). Note any changes in the  brightness of the field when you change objectives. When the object on your slide is in focus  for each objective, the distance between the slide and the objective lens, the working distance,  decreases as the objective magnification increases. Like the human eye, the lenses of your microscope provide a limited depth of focus. This means that only part of the object will be in sharp focus; areas above and below that  plane will be slightly out of focus or not in focus at all. To visualize 3-dimensional form and the concept of depth of focus, place a commercially  prepared slide on your microscope in which 3 differently colored threads have been mounted  in such a way that they cross over one another. (Note that you do NOT have to place water  and a cover slip on a commercially-prepared slide.) Using the lowest power objective, focus  on the point at which the strands intersect and determine the depth of focus at this magnification. Switch to a higher power objective. At this higher magnification, it is difficult but not impossible  to determine 3-dimensional form. You can do this by building a series of optical sections in  your mind as you focus through the specimen. Try to determine the 3-dimensional structure of your preparation at high power by  visualizing a series of optical sections. Begin by focusing on the surface of the top thread and  work through to the lower surface of the bottom thread. What color thread is on the top?_____________________ What color thread is in the middle?____________________ What color thread is on the bottom?____________________ The approximate size of a specimen can be estimated by comparing it to the diameter  of the field of vision. The field of vision is what you can see when looking through the  microscope. To estimate the diameter of the field of vision take a 15cm, transparent ruler and focus it  under the scanning objective. Align the edge of the ruler with the edge of the field of view.  Then measure the width of the field of view. Remember that the increments of the ruler is in  millimeters. So count the number of lines from left to right of the field of vision. If the field ends  between two lines do your best to estimate a fraction. What is the diameter of the scanning field of view in millimeters?______________________ What is the diameter of the low power field of view in millimeters?____________________ What is the diameter of the high power field of view in millimeters?___________________ Even with sufficient magnification and resolution, you can visualize an object under a  microscope only if there is sufficient contrast between the object and its surroundings or  between the various parts of the object. e. Contrast d. Magnification and Measurement c. Depth of Focus b. Brightness of the Field of Vision and Working Distance