238 



The Microscopic Compass. 



of brass, three inches in diameter and very light, weighing fifty grains. 

 Although the word "card" is inappropriate, yet as it has been "coin- 

 ed" by use for that part of a compass when it moves with the needle, 

 I shall use it in the subsequent part of this paper. 



Fig. 2. 



Fig. 3. 



Fig. 4. 

 K 



IJH 



This card is suspended in a brass box three and a quarter inches 

 in the inside diameter and half an inch deep, (TV Fig. 2.) The 

 sights (K S and V J Fig. 2.) arise from opposite sides of the box as in 

 the surveyor's compass, but are not carried out on arms, the distance 

 between them being the exact diameter of the box. Indeed these 



