DARK-GROUND ILLUMINATION 



415 



object. This is a lateral lesson on the value of having the bull's- 

 eye fixed to the lamp, so that both may be moved together ; and 

 there should be a notch in the slot or arm which carries the 

 bull's-eye to denote when the flame of the lamp is in its principal 

 focus. 



The above are fundamental principles of illumination, and if the 

 student is to succeed as a manipulator he must demonstrate and re- 

 demonstrate them, and become master of their details and what they 

 collaterally teach. P--- 



We may, however, with much advantage give them a larger and 

 more detailed application to the practical setting up of a dark- 

 ground illumination, as in fig. 356. 



Let an object such as a tricerdtinm (diatom) be taken, and sup- 

 pose that the objective employed is a ^-inch of *28 N.A. We must 

 first adjust the lamp and bull's-eye, as in fig. 344, and get the edge 

 of the lamp flame extended to a disc as at A. 



Xow let a small aperture be put into the condenser and a tri- 

 ceratium on the stage and the objective on the nose-piece. 



The microscope being put into position, the lamp should be 

 placed on the left-hand side of it a lamp with a fixed bull's-eye is 



FIG. 360. 



FIG. 361. 



FIG. 362. 



FIG. 363. 



assumed and it should now be arranged as to height, so that the 

 rays from the bull's-eye should fall fairly on the plane mirror, this 

 latter being inclined so as to reflect the beam on the back of the 

 sub- stage condenser. 



Xow, with any kind of light, focus, and place in the centre of the 

 field, the triceratium, as in fig. 360 ; then rack the condenser until 

 the small aperture in its diaphragm comes into focus ; centre this 

 to the triceratium,^ in fig. 361. Rack the condenser closer up 

 until the bull's-eye is in focus, as in fig. 362. 



Here it happens that the bull's-eye is not in the centre, and it is 

 not uniformly filled with light, but has instead two crescents of light 



This is a case which frequently repeats itself, but it is of course 

 not inevitable. The bull's-eye may be more or less filled with light, 

 and may or may not be more nearly centred. In this case we have 

 next to centre the image of the bull's-eye to the triceratium by 

 moving the mirror, as in fig. 363. 



But it will be noticed that this centring of the image of the 

 bull's-eye does not rectify the diffusion of the light. This will be at 

 once done by moving the lamp with attached bull's-eye ; this motion 

 requires to be a kind of rotation in azimuth round the wick as an 

 axis. The relative positions of the lamp and bull's-eye must on no 



