30 THE FROG 



which is placed upon the stage of the instrument (Fig. 25). The lenses 

 that are attached to the nosepiece at the lower end of the tube, or near 

 the object to be studied, are known as the objectives. There are two 

 objectives, the shorter of which is a low-power objective and magnifies 

 less than the longer or high-power objective. Since the nosepiece can 

 be rotated either of these objectives can be brought into line with the 

 tube. In addition to the objectives there are two other lenses, the 

 oculars, or eyepieces, either of which can be inserted into the upper 

 end of the tube or the end near the eye. The longer or low-power 

 ocular magnifies less than the shorter or high-power ocular. The tube 

 is raised or lowered with reference to the stage, and the object thereby 

 brought into focus, by means of the adjustment screws, of which there 

 are two. One of these moves the tube conspicuously and is called the 

 coarse adjustment; it is used only when the low-power objective is in 

 line with the tube. The other moves the tube very slightly and is 

 called the fine adjustment; it is used with the high-power objective 

 and to get a very sharp focus. 



(c) In order to see a highly magnified object it is necessary to 

 have it well illuminated. For most of your microscopic work the nec- 

 essary illumination is obtained by transmitted light, that is, by light 

 that passes through the object of study. The parts of the microscope 

 that are used for illumination are located below the stage. The mirror, 

 which is flat on one side and concave on the other, can be moved in 

 any direction in order to reflect light from the sky or from a lamp 

 through the hole in the stage. In many microscopes there is a set of 

 lenses constituting the condenser between the mirror and the stage. 

 This is a device for increasing the number of light rays that pass from 

 the mirror through the object. If your microscope has no condenser 

 and is near the source of light, use the fiat surface of your mirror; 

 otherwise, the concave surface will give better illumination. Some- 

 times, as with very transparent objects, it is desirable to reduce the 

 amount of light, and it is always necessary to have brighter illumina- 

 tion when working with the high-power objective. The iris diaphragm 

 is a shutter by means of which the degree of illumination can be 

 varied. Find the lever by which it can be opened and closed. 



(d) Objects to be studied with the compound microscope are 

 mounted, either temporarily or permanently, on glass slides. Make a 

 temporary mount of a small piece of paper ruled in millimeter squares. 

 Clean a slide and a cover glass with a soft cloth; it must not be for- 

 gotten that success with microscopic work depends largely on clean- 

 liness of lenses, slides, and cover glasses. Place the piece of paper 

 near the middle of the slide and cover with a drop of water. Pick up 



