ANIMAL AND VEGETABLE 239 



poles of the magnets. The cover is then replaced and 

 clamped on with the screws provided for the purpose. 



THE SCALE. 



It is clear that a light must be thrown upon the mirror of 

 the galvanometer and reflected back upon the scale. There 

 are two ways of doing this. One is to have the direct 

 light at the back of the scale, thus 



Fig. 127. 



This is a cheap pattern of scale, but is quite useful for all 

 purposes where the observer can place himself close to it. 

 In testing the human body, however, the positions of the 

 galvanometer, the scale, and the patient in relation to the 

 observer have to be considered, and it will be evident that 

 with the patient several feet away from the scale the 

 observer must be at some disadvantage. To obviate this 

 difficulty it is better to have a transparent scale (Fig. 128). 



It has a mirror upon a universal joint. The lamp faces 

 the same way as the galvanometer. Its light is thrown 

 upon the scale, reflected therefrom upon the mirror of the 

 galvanometer, and thence back to the scale. The height 

 of the scale is adjustable, and there is a ratchet arrange- 

 ment to move the scale itself some inches to get a true zero. 



