42O MANIPULATION AND PRESENTATION OF THE MICROSCOPE 



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

 justing 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 performing 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 be 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 that 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. 



13th. 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, 

 ;ire seen very well when mounted dry on cover by means of a J-inch 

 objective and a Lieberkuhn ; the bull's-eye and the plane mirror should 

 be used. Some 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 destroying 

 the contrast. It will illustrate this w^hen we recall that dirt on an 

 eye-piece which is quite 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 

 with a J-inch objective; for a general view of ' pond life' a IJ-inch 



