DETERMINATION OF MAGNIFICATION 173 



the image obtained by the actual size of the stage micrometer 

 rulings gives the magnification for the objective used. If an 

 ocular was in place the value found will be for the particular 

 combination of objective and ocular selected. 



Instead of employing a screen upon which to project the image 

 of a scale, we may use a drawing camera, thus projecting the 

 image upon the page of a note-book so raised above the plane 

 of the table top as to be at the standard distance, 250 milli- 

 meters, measured from the edge of the reflecting prism to the 

 surface of the paper in the line of the light rays, as indicated 

 in the diagram, Fig. 64, by the line abc. 



In order that measurements of magnification made by dif- 

 ferent observers may be comparable and that tables of magni- 

 fication published by microscope makers may be properly inter- 

 preted; magnifications are always recorded for the standard 

 distance of 250 millimeters which is the distance of most distinct 

 reading vision of the normal human eye. 



In photography the distance between the sensitive plate (or 

 ground glass) and the object is frequently changed to suit the 

 requirements of the particular case in hand. To determine the 

 magnification obtained in the photograph substitute a stage 

 micrometer without disturbing in any other way the adjust- 

 ment of the instrument. The projected rulings of the microm- 

 eter are measured on the ground-glass of the camera with a 

 pair of dividers or a suitable fine scale. Dividing the size of 

 image of these projected rulings by their true value gives the 

 magnification of the photograph. It will be found a great con- 

 venience for future reference to carefully scratch, or draw with 

 ink, upon the negative, the size of the projected image of the 

 stage micrometer, as indicated for example in Fig. in. 



It is evident that any note-book record of the magnifying power 

 of the various possible combinations of oculars and objectives 

 must be accompanied by a record of the tube-length employed 

 in the measurements. For this reason in determinations of mag- 

 nification it is best to use the tube-length for which the objectives 

 and oculars have been corrected. It is also evident that the 

 paper BB and the reflecting mirror M must be so placed that 



