84 



watching carefully for results. If this has no influ- 

 ence, return it to its number or to a point where the 

 outline of the object appears most sharp. Now look 

 to the illumination ; vary the distance of the mirror 

 to the object, and, if it conflicts with the stage or 

 does not give the desired results, vary the distance of 

 the lamp to the instrument and watch the effect of 

 the change through the tube. A great change in the 

 illuminating power can thus be produced ; the light 

 is best when it covers the least space, as it is then 

 most intense. The light may be quickly adjusted by 

 throwing it upon a point on the slide in the opening 

 of the stage and watching it there: 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 to the former, 

 and close to it so that the light is thrown on the lat- 

 ter. 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 it a little below 

 the line of the face of the stage, so that the light will 

 not strike it on its upper and as little as possible on 

 its lower surface ; if the light from the bull's-eye 

 directly reaches the object, it destroys the effect of 

 the oblique illumination. 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 



