420 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



object on the stage, it must be centred by means of the sub-stage ad- 

 just ing screws. 



7th. Rack up the condenser until the image of the flame comes 

 into focus. 



8th. Centre the image of the flame to the object on the stage by 

 moving the position of the lamp, and place the lamp so that the 

 edge of the flame is presented. In pel-forming this adjustment the 

 sub-stage centring screws must on no account be moved. (If a mirror 

 is employed, the centring of the image of the flame upon the object 

 can lie effected by moving the mirror.) 



9th. The object to be examined may now be substituted for that 

 used for centring purposes, and be placed in the image of the edge 

 of the flame. 



10th. The objective by which the object is to be examined is 

 placed on the nose-piece and the object brought into focus. 



llth. The eye-piece is removed and the back lens of the objective 

 is examined. The diaphragm at the back of the condenser is then 

 altered so thai three-fourths of the back lens of the objective is filled 

 with an unbroken disc of light. 



12th. The eye-piece is replaced and the objective brought into 

 adjustment either by screw collar or by altering the tube length. 



l.'lth. If it is necessary. at any time to use a large field for a rough 

 survey of an object, or to localise any particular portion of an 

 object, all that is necessary is to rack down the condenser until the 

 whole field becomes illuminated ; but when any part requires critical 

 examination the condenser must be racked up again and the image 

 of the edge of the flame focussed on the object. 



For learning the manipulation of the instrument no class of 

 objects are as suitable as diatoms ; they are also an excellent means 

 of training the eye to appreciate critical images. For a general view 

 of the larger diatoms take a spread slide in balsam ; a ^ of 80, a 

 good binocular, and a dark-ground illumination will give a fine effect. 

 This is not merely a pretty object, but it is also a very instructive 

 one, because we obtain a far clearer idea of the contour of various 

 diatoms than can be obtained in any other way. The diatoms should 

 'be studied and worked at in this manner most carefully and for a, 

 long time. The same identical specimens should be then viewed with 

 transmitted light. This lesson, if conscientiously learnt, will teach a 

 student how to appreciate form by focal alteration. This is a most 

 important lesson, and, if several days are spent in mastering it, they 

 will be far from thrown away. Diatoms, especially the larger forms, 

 sire seen very well when 'mounted <//</ <>n corer by means of a j-inch 

 objective and a Lieberkiihn ; the hull's eye and the plane mirror should 

 lie used. Home objects are so transparent, or become so transparent 

 in the medium in which they are mounted, that they will not bear a 

 large illuminating cone, the brightness of the illumination desi roving 

 the contrast. It will illustrate this when \\ e recall that dirt on an 

 '\v piece which is (jiiite invisible in a strong light becomes im- 

 mediately apparent in a feeble light. Thus animalcules require a 

 small cone of illumination when they are being examined, particularly 

 \vith a {-inch objective; for a general view of 'pond life' a Ijj-inch 



