1902.] 



A Portable Telemeter, or Range-finder. 



355 



(11.) Errors of the Binoculars. — If the binoculars be not held level r 

 the plane in which the measurement of angles is made may be inclined 

 to the plane of vision. The error amounts only to the versed sine of 

 the angle of inclination, and to produce an error of only 1 per cent, in 

 the distance, this angle would have to be at least 8°. The binocular is 

 liable to an index error, which can be corrected by setting the index of 

 the scale if its amount be known. This error would be a constant 

 source of trouble if the instrument were not strongly made. Even so, 

 however, the index must be set whenever the distance between oculars 

 has been changed for a new observer. To test the index of the 

 •binocular, choose any object of unknown distance. First use the 

 instrument without the base, and move the index to read infinity. 

 Then use the base. Suppose the distance recorded to be 3000 yards. 

 Deduct l/30th, because the distance between the objectives is 1/30 of 

 the whole base. Set the index so as to read 2900 yards; the index 

 error is then corrected for all distances. Temperature may have an in- 

 fluence on the binocular by expanding to different extents the metal 

 connecting the object glasses and the metal connecting the balloons 

 in the focal planes. Most of these metal connections are a solid brass 

 hinge which cannot be subject to variations of temperature in its parts. 

 But the other parts of the connections, in the bodies of the tele- 

 scopes, might vary one or two degrees in temperature. The distance 

 between the centres of the objectives or balloons, independent of the 

 hinge, is about 25 mm., but cannot be measured accurately. With 

 brass 25 mm., each degree Centigrade expands it 0*0005 mm. Now 

 the focal length of the objective is 143 mm., and the angular permissible 

 error being 0-000014, the difference between the distances by which 

 the two objectives and the two balloons are separated must not vary 

 to the extent of 143 mm. x 0*000014 = 0*00200 mm. due to 4° C.. 

 This difference of temperature is not to be expected in practice. In 

 cases where great distances may have to be measured with the utmost 

 exactness the index error should be corrected, in the manner described 

 above, at the time of observation, and this will correct for tempe- 

 rature. To do this easily the binocular is mounted to the base by a 

 hinge, which enables the base to be removed during the preliminary 

 observation with a minimum of time and trouble. 



(12.) Practical Adjustments. — The fields of view of the binocular are 

 limited by two round diaphragms. The outer prisms show a square 

 field with ragged edges in the round field. The hinge of the base and 

 the binocular mounting are so set by the maker that each square field 

 is in the middle of its round field horizontally. They may be above 

 or below, and they may be twisted about the visual line, and these 

 corrections are separately made by the two adjusting screws on each 

 of the two middle prisms. It is best to make all adjustments first on 

 the half-base to which the binocular is attached ; and afterwards on 



