xii GENERAL DIRECTIONS 



g) If the object is very transparent, reduce the amount of light before begin- 

 ning to focus, because too much light will make it invisible. Moving the slide 

 slightly while trying to focus will facilitate the process. 



5. To use the high power: 



a) The object must always be under a cover glass when the high power is to 

 be used. 



b) The high power cannot be used with thick or thickly mounted objects. 



c) The object to be viewed must always be found first with the low power. 

 Never try to examine anything first with the high power. Place the object or part 

 of the object which you desire to study with the high power in the exact center 

 of the low-power field. 



d) Swing the nosepiece around so that the high-power objective comes into 

 place. 



e) The object should now be nearly in focus and is brought into exact focus 

 by means of the fine adjustment. If the object is not in focus when the high 

 power is swung into place, the microscope is not perfectly adjusted and the 

 following procedure must be followed: 



(1) The center of the field of the high power may not be the same as the 

 center of the field of the low power. The object will therefore not be in the field 

 at all when the high power is swung around. The remedy is to find out where the 

 center of the high-power field is on the low-power field and to place the object 

 there instead of in the center when preparing to use the high power. 



(2) The focus of the high power may not be the same as the focus of the low 

 power. This is the common difficulty. The remedy is to screw the tube up 

 or down after the high power is in place until a focus is obtained. You will 

 have to find out by trial whether to screw up or down. If up, then always 

 remember to raise the tube of the microscope before swinging the high power 

 into place, as otherwise the objective will strike against the slide. 



(3) Have the assistant help you find out the peculiarities of your microscope. 

 /) After getting a focus, adjust the amount of light by means of the diaphragm. 



The amount of light best for the high power is never that best for the low power. 

 g) Note how close to the slide the high-power focuses. For this reason thick 

 objects cannot be viewed under the high power, and care must always be taken 

 not to run the objective into the slide, as this will break the slide and may injure 

 the lens. 



6. Plane of focus. As the microscope is an optical instrument, the planes 

 of focus of its lenses are geometrical planes, i.e., planes without thickness. All 

 objects viewed through the microscope have an appreciable thickness. It is 

 therefore obvious that no object, no matter how thin it is, can be seen in its totality 

 in a single plane of focus, as some parts are certain to lie outside that plane. 

 It is therefore necessary to change the focus continually while mewing an object in 

 the microscope. This is particularly essential when using high powers. Students 



