nothing more aggravating than a lack of stability. 

 A considerable weight directly under the pillar is of 

 little value, and a great expansion of the resting points 

 with extreme thinness is little better. There should 

 be a combination of both qualities and if suitable pro- 

 portions are not maintained, an excess can hardly be 

 called a fault, whereas too little would certainly be. 



Joint for Inclination. This should work 

 smoothly but firmly, and the arm should remain in 

 any position in which it is placed. If it has a gritty 

 sensation, the two parts are liable to " eat " and finally 

 reach a point where they cannot be moved. 



Besides the above qualification a good joint should 

 work without the slightest back-lash when the arm is 

 worked quickly back and forth through small arc. 



When the arm comes against the stop for upright 

 position it should not lean forward. 



Mirror and Mirror-Bar. The proper illumina- 

 tion of an object is an important feature and although 

 there are numerous accessories for properly accomplish- 

 ing this, the mirrors alone are effective agents when 

 properly constructed and applied, particularly when no 

 high magnification is used. The plane mirror is 

 generally used with very low powers and reflects light 

 in about its original intensity. The concave mirror, 

 however, is intended to concentrate the light so that 

 all the rays which strike its surface are reflected and 



