624 



mirror strips securely to the wood wedge, 

 being very careful not to disturb the adjust- 

 ment of the strips. Make the sealing wax 

 very hot with the nail so that it will not 

 break loose. 



The other part of the work is easy. Cut 

 from a board a piece measuring 8 in. long, 

 2 x /l in. wide and Yi m - thick, and nail a 

 wood strip of ^-in. square cross-section 

 across one end at a 45 deg. slant. Glue the 

 wood wedge against this strip so that the 

 mirrors stand as in B, facing away from the 

 supporting strip. 



To complete the range-finder cut a piece 

 of tin 1% in. square. Cut into it a slit 1 in. 

 long and 1/64 in. wide and tack the tin 

 against the end of the baseboard so that the 

 slit and the thin edge of the wood wedge are 

 parallel and the same distance from the 

 side edge of the board. 



The distance of an object, C, may be 

 determined as illustrated. Turn the left 

 cheek toward the distant object and, look- 

 ing straight ahead, hold the slit of the 

 range-finder close up to the left eye and 

 look through the slit into the mirrors. 

 Point the instrument in such a direction 

 that the image of the distant object, as 

 seen in the upper mirror, lies exactly above 

 and directly in line with the thin edge of 

 the wedge. At the same instant, glance 

 past the wedge-edge and note some feature 

 in the landscape which is in line with this 

 edge, a tree, D, for instance. Now with 

 the eye at the slit walk directly toward this 

 object, D, counting the number of paces 

 taken, still keeping the wedge-edge in line 

 with the tree. Meanwhile the image of C 

 in the upper mirror moves on out to the 

 right and at the same time its image as seen 

 in the lower mirror moves up from the left 

 until it in its turn comes into line with the 

 wedge-edge. At the instant this happens 

 note how many paces, E, have been taken 

 from the first observing station. Multiply 

 this number by 10 and you have the number 

 of paces, F, from the first point of observa- 

 tion out to C. If you have learned to 

 make a stride of 36 in., then the distances 

 are known at once in yards, otherwise you 

 must know the length of the natural stride 

 before you can obtain distances in yards. 

 Of course the paced distance can be 

 measured with a tape-line. If the paced 

 distance is 88 yards then C is 880 yards or 

 half a mile away. 



It is evident that to measure with this 

 instrument the distance to an object 5 miles 

 away one must pace off half a mile. This 



Popular Science Monthly 



is sometimes inconvenient. It is therefore 

 desirable, for objects at this distance, or 

 farther away, to use a range-finder whose 

 wedge is of only half the angle described 

 above; the 2-in. paper square must then 

 be held at a 40-in. distance in adjusting the 

 mirror strips. The paced distance is then 

 to be multiplied by 20. 



This type of range-finder yields astonish- 

 ingly accurate results if it is equipped with 

 good mirrors. It is a scientific instrument 

 based upon the laws of reflection of light 

 and surveyors' triangulation methods and 

 is worthy of any boy's serious consideration. 



A Convenient Pivoted Card File 

 for the Desk 



FILING systems are necessary in all 

 lines of business, and for convenience 

 of small accounts or for data a single tray 

 is often applicable. The desk tray illus- 

 trated is especially designed and it fits 

 into a slide made for it, so that it is never 



A desk card file tray pulls out the same as 

 a drawer and turns on a pivot when drawn 



in the way. Another feature of the tray 

 is that it turns on a pivot and the cards 

 face the person sitting at the desk. 



Flanged bottoms hold the guide cards in 

 place. There is a follower-block, simple 

 in construction and operation, that holds 

 the tabs in a readable position. Side tabs, 

 as shown at A, permit more distinctive 

 subdivisions. With each closing of the 

 drawer an automatic angle-block tilts the 

 cards back parallel with the follower-block. 

 To further facilitate reference the index 

 drawer is pivoted to its carrier-shelf. 



The frame of the drawer is made up of 

 two thick ends to which thin bottom and 

 side pieces are nailed, the exact dimensions 

 of which depend upon the size of the 

 cards as well as upon the materials at hand. 

 To the front end block, planed to serve 



