44 ANNUAL OF SCIENTIFIC DISCOVERY. 



ordinary telescope, having attached to its eye-tube at the inner end 

 another tube of equal dimensions, and divided throughout its whole 

 length by a vertical partition. At the end of this tube next the eye- 

 piece is placed a ring containing a bisected lens, the two halves of 

 which are equally inclined on opposite sides of the vertical plane, per- 

 pendicular to the axis of the telescope. The eye-tube is itself divided 

 by a vertical diaphragm, which abuts against this system of semi-lenses 

 and is prolonged very nearly to the eye-piece. In adjusting the in- 

 strument, these two diaphragms must accurately coincide in the same 

 plane, which is arranged by turning the screw which connects the two 

 tubes. The target-staff has two targets projecting at right angles to 

 it on opposite -sides, the lower half of each being colored black, and 

 the upper wiiite. The upper target is stationary, and from its centre 

 line a graduated scale proceeds downwards along the staff as far as is 

 necessary. The lower target slides upon the staff, and carries an in- 

 dex opposite to its centre line, which indicates the degree of gradua- 

 tion to which its position corresponds. When in operation, the rays 

 from the upper target, which fall upon the object-glass, will form an 

 image of the target in the instrument, from which image the rays will 

 strike upon the two inclined lenses, so as to form an image above or 

 one below the axis ; unless, however, the inclination of the lenses to 

 the vertical plane perpendicular to the axis be very small, these two 

 images will be too far apart to be both in the field of the instrument at 

 the same time. Suppose only the lower one is visible. The lower 

 target will also produce two images, of which the upper one will be 

 in the field of the instrument, and by sliding the lower target up or 

 down we can find a position in which the images of the two targets 

 coincide, and their central lines then appear as one horizontal line 

 crossing the staff. Now, by the mathematical theory of the refraction 

 of light through lenses, when this is the case the distance between 

 the lines will be so nearly proportional to the distance of the target 

 from the instrument, that the error in assuming it to be really propor- 

 tional is so small that it may be overlooked, especially as it dimin- 

 ishes with the increase of the distance. The accuracy of this instru- 

 ment depends upon the care exercised in determining the exact point 

 of coincidence of the two lines, and on the precision in reading the 

 scale. The rapidity with \vhich the work is done is very great, and 

 the committee think that in rocky, bushy, or marshy grounds, it will 

 be of great use, as well as where distances across sheets of water are 

 to be measured. 



SELF-DETERMINING VARIATION-COMPASS. 



THIS instrument the invention of .1. R. St. John is the admira- 

 tion of all our scientific and literary men who have seen its opera- 

 tions, and it received the gold medal of the American Institute last 

 year. It is of the simplest construction, and while it does not inter- 

 fere with the use of the compass as now practised, it merely gives an 

 addition by which the deflections of the needle, from whatever causes 

 produced, are accurately marked, and the corrections from the true 

 geographical meridian at all times shown by simple inspection. 



