10 INTRODUCTION. 



time, with the right eye, look at a centimetre scale held close to the 

 stage and at a distance of 25 cm from the eye. The two scales appear 

 superposed upon each other. Count the stage divisions which exactly 

 correspond to one cm. of the centimetre scale, which, with the high 

 power we will presume to be 3,0'03 mm now appear equal to 10 mm and 

 are therefore magnified 333 times, and this is the magnifying power of 

 the combination. To measure an object, compare its magnified image 

 with the millimetre scale and compute from the ascertained ratio. 

 Instead of the mm. scale, the points of a pair of dividers may be held 

 in the required position and be made to include a number of the stage 

 divisions. The distance between the divider points is then read off on 

 the cm. scale, the computation being the same as before. 



