408 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



which the bull's-eye (with a catch fixing its focus from the flame) 

 were so affixed as to be carried up and down and to right and left 

 with the lamp. 



When the microscope is fixed in its upright position, and the 

 prism is arranged to give direct and not oblique reflexion, the lamp 

 flame, by means of a card, is arranged as nearly right for the re- 

 flexion of the image of the flame into the centre of the field as may 

 be, and then a little movement in one or both milled heads will 

 bring it accurately into the field. 



We may arrange the microscope for ordinary transmitted light, 

 that is, for light caused to pass through the object into the object- 

 glass, by placing it upon the table, arranged as already directed ; 

 the instrument is then sloped to the required position, and a con- 

 denser, suitable to the power to be employed, 1 is put into the sub- 

 stage. The lamp is now put into the right position, with a bull's- 

 eye, on the left of the observer. The condenser is then, as described 

 below, to be ' centred,' when the objective may be changed 

 as desired, and the eye-piece altered to suit. But it should be 

 carefully noted that, if apochromatic powers are being used, there 

 must be accurate adjustment of the tube length if the best results 



are to be obtained ; and 

 with any serious increase 

 of the power of the objec- 

 tive a condenser of higher 

 aperture and shorter focus 

 must be used. 



FIG. 344. Edge of lamp flame in centre and Often, however, as good 



focus of bull's-eye. or better results may be 



obtained without the em- 

 ployment of the mirror at all, the light being sent directly through 

 the condenser from the lamp flame. The mode of arrangement for 

 this kind of manipulation is presented in Plate V., where it will be 

 observed that the microscope is inclined more towards the horizontal 

 to suit the observer ; the lamp is directly in front of the sub-stage, 

 the mirror is turned aside, and a frame (fixed upon a bull's-eye 

 stand) carrying a monochromatic screen is placed between the lamp 

 flame and the condenser (sub-stage). 



By this means the light is sent into the condenser and upon the 

 object, and is then treated as is the case (for centring) when the 

 mirror is used. The first step in the direction of efficiency in the 

 use of the microscope is to understand the principles of illumination, 

 and a knowledge of the various effects produced by the bull's-eye 

 lies on the threshold of this. 



Having given details as to the forms of lamp which are of most 

 service, we assume that a paraffin lamp with 1-inch wick is used. 



If we place the edge of this flame (E, fig. 344) in the centre and 

 exact focus of the bull's-eye B, A shows the effect of doing so. 



If a piece of card were held in the path of the rays proceeding 

 from B, the picture as shown at A would not be seen instead of it 

 an enlarged and inverted image of the flame. The image at A is 

 , x Vide. Chapter IV. p, 298. 



^ A 



D 



