140 



MAGNIFICATION OF THE MICROSCOPE 



[Ch. V 



always understood that this magnification is at the standard distance 

 of 250 mm. The necessity for the adoption of some common stand- 

 ard will be seen at a glance in fig. 85, where' is represented graphi- 

 cally the fact that the size of the virtual image depends directly on 

 the distance at which it is projected, and this size is directly propor- 

 tional to the vertical distance from the apex of the triangle, of which 



it forms a base. The 

 distance of 250 milli- 

 meters has been 

 chosen on the sup- 

 position that it is the 

 distance of most dis- 

 tinct vision for normal 

 adults when examin- 

 ing details. 



In preparing draw- 

 ings it is often of great 

 convenience to make 

 them at a distance 

 less or greater than 

 the standard. In that 

 case the magnification 

 must be determined 

 for the image distance 

 actually used. 



237. Magnification and relation of the object to the principal 

 focus. — As shown by figures 86 and 87, independent of the equivalent 

 focus of the simple microscope or the objective, the real image or the 

 virtual image, as the case may be, will be larger the nearer the object 

 is to the principal focal point. 



In figures 88 and 89 it is shown also that if the object or the real 

 image is in the plane of the principal focus, the rays emerging from 

 the simple microscope or the ocular will be in parallel bundles, and 

 when projected by the eye must also be in parallel bundles. It is 

 further shown in such a case that the rays emanating from any point 

 in the object or real image will not in that case form a virtual point 



Fig. 85. Diagram to Show that the Size of 

 the Virtual Image Depends upon the Projec- 

 tion Distance. 



a Size of image at a projection distance of 25 cm. 



b Image at 35 cm. 



The sizes are directly as the projection distances. 

 C The camera lucida and under it a spectacle lens 

 to aid the eye in focusing the pencil point; this is 

 only needed by those with defective eyes. 



