112 ARRANGEMENT OF APPARATUS. 



The lamp should now be brought near the stand, 

 on which it rests, (i.e., as low as possible), by the 

 rack and pinion movement attached to its back. 



Our reason for keeping the lamp low and the 

 mirror high is the greater the angle made by the 

 incident and reflected rays, the better the illumina- 

 tion. 



The light from the lamp may be thrown on the 

 condenser by altering the inclination of the mirror, 

 the eye at the same time being applied to the eye- 

 piece. 



Still looking through the microscope, the pin hole 

 cap is brought into focus, and centred, by turning 

 the screws in the rim of the substage tube. 



When central, the diaphragm, plate is turned till 

 a medium-sized hole is under the condenser ; this 

 aperture is now brought into focus by increasing the 

 distance of the objective from it, and made coinci- 

 dent with the optic axis of the instrument, by dis- 

 placing the diaphragm wheel sufficiently in its own 

 plane. 



This being accomplished the flame of the lamp is 

 focussed, by altering the distance between the top of 

 the condenser and the objective, (in other words by 

 using the coarse or fine adjustments of the micro- 

 scope), and centred by altering the inclination of 

 the mirror. We now make the image of the flame 

 of the lamp itself, and that of its reflection in the 

 mirror placed behind it, coincident. This may be 

 done by giving a slight twist or tilt, as the case may 

 be, to the tin cover surrounding the lamp. The 



