240 The Microscopic Compass. 



ly great distance. On account of the focal adjustment of the eye both 

 cannot be clearly seen at once. 



Any person may be satisfied of this by making several marks the 

 twentieth of an inch apart on the edge of a piece of paper, holding it 

 up at the distance of three inches, and looking over the graduated edge 

 at a distant object ; the lines will appear entirely blended ; if the at- 

 tention be then directed to the lens the object disappears or is seen 

 very indistinctly. I mention this principle more especially, because 

 I once saw a French compass of this defective construction, in which 

 the sight and the reflector were close to the card. It could not of 

 course be used for objects either above or below the horizon and even 

 for horizontal objects it could not, for the above defect, be read with 

 accuracy. It was the inspection of this however which led me to at- 

 tempt the present improved invention. 



The degrees in my compass are magnified so much that each de- 

 gree becomes a true measure of a degree of the horizon, as the de- 

 grees of a horizontal circle would be if they were seen from the cen- 

 tre. When a degree of a circle is viewed from the circumference, it 

 measures one-half a degree; Euc. iii. 20. If under these circum- 

 stances the degrees were magnified twice, then a degree of the circle 

 seen at the distance of the diameter, would be the true measure of a 

 degree. If the degrees are seen at a greater distance than a diame- 

 ter, they become proper measures of degrees by being proportionally 

 magnified. This is true only of a few degrees on each side of the 

 diameter; so far only as the arc, chord, sine and tangent have no sen- 

 sible difference. The degrees of the card in my compass are mag- 

 nified very nearly to that proportion, so that the bearing of all the ob- 

 jects in the field of view, which takes in ten degrees, can be read at 

 once without moving the instrument : Fig. 4 exhibits the appearance 

 of the field of view as seen in the reflector. The space D G includes 

 ten degrees of the card seen in the silvered part ; D H is the part of 

 the horizon seen through the unsilvered part. The bearings of the 

 objects A, B, C, would be read as follows, A 177°, B 179° 25', 

 C 184°. It will be seen from this reading that the circle is number- 

 ed quite round from to 360. Zero or is placed North and the 

 reading continued eastward. East is 90°, South 180° and West 

 270°. Reading at several points in the field of view as above is lia- 

 ble to error from the vibrations of the card up and down and from 

 " aberration" of the lens, the degrees being seen out of its axis. A 

 principal reading point or sight is therefore made by a scratch or line 



