Light is either transmitted or reflected. When the 

 former, it is used to illuminate transparent objects and 

 passes through the objects from below the stage into 

 the objective. In opaque objects this is impossible 

 and reflected light is required, when it is directed onto 

 the object from above and illuminates its upper surface. 

 In the following instructions it is assumed that trans- 

 mitted light is used unless otherwise stated. 



The concave mirror converges the light and there- 

 fore has a focal point, and it is evident that if its focus 

 is of such length that with parallel rays (daylight) it 

 will fall on the object, the focus will be longer with 

 diverging rays (lamplight) and when no provision is 

 made in the instrument to adjust the mirror to meet 

 these two conditions, it becomes difficult and some- 

 times impossible in critical work to obtain the best 

 results. For this and other advantages an additional 

 illuminating apparatus, called a condenser, is now com- 

 monly used with medium and high power objectives. 



Before lighting an object make certain that the 

 mirror-bar is in exactly central position and set the 

 mirror at such an angle to the light that it will be 

 directed upon the object, which can be done most 

 quickly at the outset by observing the object directly, 

 keeping the head at one side of the tube. Now 

 remove the eyepiece and observe the light through the 

 objective. It should be central and of equal intensity, 

 which with daylight is sometimes difficult to obtain as 



