ADJUSTMENT OF CONDENSER 161 



monochromatic blue, green or orange light is obtained 

 (Barnard). 



As a general rule the microscopist should employ only 

 sufficient light to render the object examined clearly 

 visible. Too much light is fatiguing to the eye and may 

 drown fine details of structure. Both eyes should, of 

 course, be kept open during microscopical examination ; 

 a little practice soon enables this to become automatic. 

 A sheet of vulcanite may be attached to the eye-piece, 

 so as to screen the eye which is not being used. 



Optical Gentration and Adjustment 



We now come to the most important part of micro- 

 scopical adjustment, viz. proper centration of the optical 

 system and illumination, which is essential for correct 

 illumination. First of all the illuminant should be placed 

 in a direct line with the microscope and adjusted as 

 regards distance, so that the light is central on the 

 mirror, the flat surface of which is always used with the 

 condenser, and will be projected up the optical axis 

 when the mirror is at an angle of about 45 degrees. In order 

 to centre the condenser it should be racked up to the full 

 extent and the |-in. objective with a low eye-piece used. 

 The iris diaphragm is contracted to a point, and by racking 

 the objective up and down the hole in the diaphragm is 

 focussed. This should be easy with a little adjustment of 

 the mirror, unless the condenser is hopelessly out of 

 centre. If this be the case the field will remain dark 

 in spite of adjustment of the mirror. In this event the 

 iris diaphragm should be opened until illumination is 

 obtained, and an edge of the diaphragm hole may then be 

 focussed, and it will be seen in what direction the con- 

 denser is out of centre. By turning the centring screws 

 of the sub -stage fitting in the right directions, the con- 



M.B. U 



