240 The Microscopic Compass. Bye 
ly great distance. On account of the focal adjustment of the eye both 
cannot be clearly seen at once. > Sees 
~ Any person may be satisfied of this by making several mat e 
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. a 
‘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. m1. 20. If under these cireum- 
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 @ 
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 sem- 
sible difference. ‘The degrees of the card in my compass aré mag- 
‘nified very nearly to that proportion, so that the bearing of all the of “f 
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 DG 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°, B179° 25 
C€ 184°. It will be seen from this reading that the circle is number- ve 
ed quite round from 0 to 360. Zero or 0 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 above is lia- gi 
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- 
principal reading point or sight is therefore made by a scratch oF 
Bare : 
