The Microscopic Compass. 241 



on the glass cover directly underneath the lens. This mark (B K Fig. 

 4) appears in the field of view crossing the card vertically. It has 

 nearly the distinctness of the degrees because it is so near the card that 

 the rays from it pass through the lens and are rendered nearly parallel. 



JManner of using the Microscopic compass. — As it is contained in 

 a box, three and a half inches in diameter and one inch and nine-tenths 

 deep, it may be carried in the vi^aistcoat pocket. 



When the bearing of an object is to be taken, remove the cover, 

 raise the sights, and holding the compass horizontally, by grasping the 

 milled edge of the bottom by the thumb and fingers of one hand, 

 move the eye up or down along the slitted sight till the object appears 

 in the unsilvered part of the reflector. With the other hand incline 

 the reflector backwards or forwards on its hinge till the graduated 

 edge of the card is brought to the lower edge of the silvered part of 

 the reflector and in apparent contact with the object. Turn the com- 

 pass round till the vertical sight line crosses the object. The degree 

 or degree and part vi^hich appears behind this line is the bearing. 

 Some advantages might be obtained by using the instrument on a 

 stand, but by a little practice the bearings may be taken very accu- 

 rately by hand. I do not propose this as a substitute for the survey- 

 or's compass ; but merely as an instrument exactly suited to ama- 

 teurs and scientific travellers, to whom it is inconvenient or unpleas- 

 ant to carry a back load of machinery, to take the bearing of an ob- 

 ject. I have, for several years, been carrying on a trigonometrical 

 survey of the beautiful valley of Cincinnati in which I reside. This 

 I have done for the recreation both physical and intellectual which 

 it affords. It invites me to exercise in the open air and is the best 

 antidyspeptic I have tried. I have managed the several points of the 

 valley very much to ray satisfaction with the sextant ; but nothing 

 answers so well for " meandering" the ravines, rivulets, and ridges of 

 the hills as the microscopic compass. I can take the angles with equal 

 accuracy as with the surveyor's and with ten times the convenience. 

 It remains to explain the figures, and make some remarks on the 

 construction. Fig. 1. represents the needle and graduated rim, a 

 vertical section of which is shown in Fig. 2. N is the needle, which 

 is one twenty fifth of an inch thick, one eighth of an inch wide, and 

 three inches long. AA are two pieces of brass, by means of which, 

 the rim is fastened to the needle by screws at BB. The pieces 

 AA have holes at BB a little larger than the screws which pass 

 through them, to allow the rim to be adjusted both to the center 



Vol. XXIII.— No. 2. 31 



