4 
but the angle BAC = T'BT=a. Let b be the base, d the distance, and y the 
parallax. Then we have 
b 
d— -cos. a—b cosec. y cos. a 
sin. y 
That is to say that the distance of an oblique object varies for the same parallax 
with the cosine of the angle a, or azimuthal angle of the object, to continue to 
borrow the convenient terms of astronomy. 
The Fiske range-finders are arranged so as to record the quantity b cosec. y, 
which, in the case of a line of sight normal to the base, is the range itself. For the 
Ordinary case of an object situated obliquely the instrument itself makes the cor¬ 
rection by multiplying the above value by cos. a. 
The Slide Range-Finder. 
This apparatus is represented theoretically by Fig. 4. At the extremities A 
and B of the base are the telescopes L and L' which slide round the conducting 
arcs h and h' and which are connected together by the Wheatstone Bridge abed , 
into the circuit of which are introduced two parallel bars, mn and mV, of 
equal resistance. Upon the bars runs a slide, of which the extremities, r and r\ 
isolated from one another, carry the terminals of the two poles of the transverse 
wire. When the slide gives electric contact to the two bars the resistances of the 
latter are divided equally between the four sides of the Bridge, and there is equi¬ 
librium if the telescope needles are parallel. But when the slide is moved, a 
variation in the resistances of the 
four sides of the Bridge is effected, 
and consequently a variation of in¬ 
tensity in the current of the trans¬ 
verse wire. If the movement of. 
the slide be regulated, the current 
developed in the transverse wire, 
by the convergence of the telescope 
needles on the object, may be an¬ 
nulled; and we can conceive the 
possibility of graduating the bars 
of resistance so that the position 
of the slide at the moment when 
that effect is obtained shall indi¬ 
cate the angle of inclination of the 
two telescopes, i.e., the parallax, or 
apex angle, required. 
In order to grasp the matter 
more closely, let us suppose that 
the bars have the same resistance 
as the arcs and are graduated in 
divisions of the same length. If 
we move the slide n divisions, we 
shall introduce into the circuit, on 
one side of the transverse wire, the 
length n of each bar, or a total re¬ 
sistance of 2n, and we can fix the 
telescope L' with a compensation 
of magnitude 2 n in regard to its needle. 
In a general sense, that is. to say, without making any particular hypothesis 
about the relative resistances of the bars and the arcs, the movement of the slide 
Fig. 4. 
