36 THE MICROSCOPE 



To centre It is therefore necessary that a centring adjustment^should 



dark'-^ound ^^ provided, SO that the illuminated point may be exactly in 

 illuminator, the field of view. If the substage of the microscope is not pro- 

 vided with a centring adjustment, the form of dark- ground 

 illuminator mount which has centring screws should be used. 



To centre the illuminator the following is a satisfactory method 

 of procedure : 



Remove the eyepiece and object glass from the microscope 

 and swing out the substage. Place the eye six or eight inches 

 above the tube of the microscope, and move the eye until the 

 lower end of the microscope tube, where the object glass screws in, 

 is central with the upper edge of the drawtube. Then, without 

 moving the eye, move the mirror until the light appears in the 

 centre of these apertures. Swing in the substage with the dark- 

 ground illuminator without moving the microscope or the mirror, 

 place a drop of cedar-wood oil upon the upper surface of the 

 illuminator, put the object to be examined on the stage, and move 

 the substage up till the illuminator is in immersion contact with 

 the sHp. This having been done, put a low-power object glass, 

 say 2/3-inch, into the microscope, use a low-power eyepiece, and 

 focus the slide. There will be sufficient dirt or particles on the 

 slide to show the small illuminated point, which will probably 

 not be in the centre of the field. By means of the centring 

 screws of the substage or the illuminator mount, this illuminated 

 patch may be brought into the centre of the field, and the 2/3-inch 

 object glass may then be replaced by the object glass which it is 

 desired to use. A further slight adjustment may be made 

 for the new object glass if necessary, after which the centring 

 screws should not be touched, but slight alterations should be 

 made by altering the position of the mirror. Until the centring 

 of the illuminator has been completed, the position of the lamp, 

 the microscope, or the mirror, should not be altered. 

 To focus Focussing, as previously mentioned, is almost impossible 



d?rk'-ground '^^^^ *^® nou-focussiug illumiuator, although a very small move- 

 iiiuminator. ment of the substage is possible without breaking the film of oil 

 between the illuminator and the slip. 



With the focussing model it is best to set the focus approxi- 

 mately by the pin on the mount, as described, but a small 

 movement of the adjusting lever while the object is being 

 observed is particularly useful in obtaining the best result. 



If the illuminator is out of focus, a dark circular patch will 

 appear in the centre of the field surrounded by a bright ring ; 

 when focussed, the central dark patch disappears. 



Objects mounted dry cannot be examined by this illuminator. 



They must be in some fluid or medium, as no light will reach the 



object if there is any layer of air between it and the illuminator. 



It is important to see that the slips and cover glasses are 



thorpughly clean and that there are no air bubbles in the oil or 



