The Use of the Microscope 



23 



Figs. 23 and 24. Setting up the high power on a freshman microscope. Fig. 23. After 

 the object has been centered with the XlO objective, the X40 is swung into position 

 and lowered, while being examined from the side, until it not quite touches the slide. 

 Fig. 24. The tube is racked up with the fine adjustment until the object is in focus. 



of between 2 and 3 in. There is also a fine adjustment which provides 

 geared-down movement of the tube over a distance of from y s to % in. 



Tube. The tube is of fixed length, usually 160 mm, and is of smaller 

 diameter at the upper end to fit the ocular or eyepiece. The lower end 

 carries a three-lens turret— still called a "triple nosepiece" by microscope 

 manufacturers— which, in cheap microscopes, usually rotates on a simple 

 cone bearing. 



Objectives. It is usual to provide microscopes of this type with X3.5, 

 XlO (16-mm, N.A. 0.3) and X40 (4-mm, N.A. 0.6) achromatic objectives. 



Ocular. One XlO ocular is all that is desirable. 



Illuminating System. No special lamp is required, and it is not neces- 

 sary to have a flat, as well as a concave, side to the mirror. 



Setting Up the Freshman Microscope. This type of equipment is per- 

 fectly adequate for teaching elementary classes in biology. It is as simple 

 to set up as it is to use, and every beginning student should be drilled 

 in the following routine, illustrated in Figs. 19 to 24: 



1. (Fig. 19.) Tilt the body back to a convenient angle, turn the XlO 

 lens into place, and adjust the concave side of the mirror until maximum 

 illumination is obtained (Fig. 20). 



2. ( Fig. 21. ) Place a slide on the stage under the clips. 



3. (Fig. 22.) Use the coarse adjustment to bring the object into focus. 



