from the instrument and watch the effect of the change 

 through the eyepiece. If neither of these changes give 

 any improvement, recourse must be had to another 

 expedient. Place a bull's-eye between the lamp and 

 mirror with the plane side of the lens toward the lamp 

 and close to it, so that the light is thrown on the mirror. 

 It should be properly concentrated, so that the circle 

 of light will not be larger than the mirror, which can 

 be determined by placing the hand or a piece of paper 

 back of it. Adjust when necessary by moving the 

 lamp or bull's-eye. Keep the mirror a little below the 

 line of the top of the stage, so that the beam from the 

 bull's-eye will not illuminate the object on its upper 

 surface. If the direct light from the bull's-eye reaches 

 the object, it destroys to some extent the effect of 

 the oblique illumination from the mirror. Great care 

 should be given to this point as it is very important. 



If all of these suggestions have been followed, a 

 great difference will undoubtedly be noticed in the 

 performance of the objective ; but if it still does not 

 come up to the standard, patience must not be lost. 

 The slightest change in the position of the mirror, or 

 bull's-eye, or lamp, or a touch to the correction collar 

 or micrometer screw, is sometimes followed by astonish- 

 ing results. The beginner should sit down with the 

 expectation that he will fail at the first trial. At each 

 succeeding trial he can easily notice his improvement 

 in manipulation and a corresponding gain in the results. 



