Appendix A: The Microscope and Its Optical Principles 155 



various combinations of objectives and oculars should be made and kept 

 for future use. On each should be recorded the tube- length used, the 

 number of the objective and of the ocular, the length of the camera mir- 

 ror-bar, and the angle of the mirror, for if any one of these is changed 

 the scale is no longer accurate. 



When much measuring is to be done an ocular micrometer is used. 

 It consists of a circular glass disk with a scale ruled on it and is inserted 

 in the ocular between the eye-lens and the field-lens. By means of a 

 stage micrometer the value of the divisions of the ocular micrometer is 

 determined for a known tube-length and every combination of lenses it 

 is desired to use in the work of measurement. Suppose that it takes four 

 divisions of the ocular micrometer to correspond to one of the finer divi- 

 sions of the stage micrometer, then since the divisions of the latter are 

 equal to T^U mm., each space in the ocular micrometer must be equal to 

 7^<r mm., that is 0.0025 mm. A filar or screw micrometer is a more conven- 

 ient form of ocular micrometer which is provided with delicate movable 

 spider lines that can be adjusted to the space to be measured by means of 

 a fine screw with very accurately cut threads (Fig. 69). At the end of 

 the screw is a graduated disk which gives the value of the distance 

 between the spider lines. The pitch of the screw is either -gV inch or 

 0.5 mm. 



Micron. The one-thousandth part of a millimeter; expressed briefly 

 by the Greek letter /A. It is the unit of measurement in microscopy. 



Mirror. The compound microscope is usually provided with both 

 concave and plane mirrors, which may be rotated or swung in any direc- 

 tion. The plane mirror is used with the condenser, the concave, when- 

 ever it is of advantage to have light concentrated upon the object with 

 the condenser out. The mirror should be capable of being moved up or 

 down the mirror-bar so that it can be accurately focused upon the object. 

 See also illumination. 



Muscae Volitantes. Small filaments or specks which float across the 

 field of vision. They are really small opacities in the vitreous humor of 

 the eye. 



Numerical Aperture. A system which expresses the efficiency of an 

 objective by indicating the relative proportion of light rays which trav- 

 erse it to form an image. With the introduction of immersion objectives, 

 it became evident that angular aperture alone is not sufficient to indicate 

 the real capacity of an objective. For instance, an immersion and a dry 

 lens may be of precisely the same angular aperture and yet the immer- 

 sion lens is more efficient because it sends more rays of light through the 

 objective (see immersion lens). It was found necessary to take cogniz- 

 ance of the medium which intervenes between the cover-glass and the 

 front lens of the objective. 



