something by moving the object under observation to the end of the pointer. 

## b. Brightness of the Field of Vision and Working Distance 

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. 

## c. Depth of Focus 

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. 

## d. Magnification and Measurement 

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?___________________ 

## e. Contrast 

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. 

