30 The Microscope 



field iris. No two slides are of the same thickness, so the substage condenser 

 must be refocused for each slide. 



The procedure for setting up the oil-immersion lens is much the same. 



1. Repeat steps 1 to 5 with the XlO objective. 



2. Swing the X40 objective into place and, if possible, find and center 

 the field to be examined under the oil-immersion lens. 



3. Rack up the tube and swing the X90 objective into place. Place a 

 small drop of immersion oil in the center of the field and, looking in side- 

 ways (compare Fig. 23), rack the tube down until the front of the ob- 

 jective just touches the oil. 



4. Focus downward with the fine adjustment until the object is in 

 focus. Shut the field iris until the field is delimited and, if necessary, 

 sharpen the image of this iris with the substage condenser. 



5. The substage iris is left wide open, so step 6 is unnecessary. The 

 N.A. of the substage condenser, since it is working in air, cannot be 

 greater than 1.0. The maximum N.A. of the objective is 1.2, so a working 

 N.A. of 1.0 is just about right. 



The only thing likely to go wrong in all this is in focusing the substage 

 condenser. If the slide lifts when the condenser is brought up, before 

 the field iris is in focus, there is nothing to be done. The slide is too 

 thick and it is necessary to be content with a second-class image. 



THE RESEARCH MICROSCOPE 



Specifications. A description of a research microscope, of which a 

 typical example with its illuminant is shown in Fig. 35, is included in 

 this elementary book for two reasons. First, such microscopes are com- 

 monly set up for class demonstrations, and the student should certainly 

 understand something about any instrument that he may look through. 

 Second, there is a real justification for the use of these complex in- 

 struments even in elementary photomicrography, the subject of the next 

 chapter. A first-class research microscope usually has most of the follow- 

 ing features. 



Stand. There are as many kinds as there are manufacturers of research 

 microscopes. The stand carries the tube, the stage, the substage, and the 

 mirror. 



Tube. Research microscopes, almost without exception, have inter- 

 changeable vertical monocular and inclined binocular bodies. The former 

 is used for photomicrography and the latter for visual observation. The 

 trinocular body, which permits simultaneous observation and photography, 

 is a recent introduction of the American Optical Company and is dis- 

 cussed in the next chapter (Fig. 66). In most research microscopes the 



