On {HEAD ~ either side} of {TAIL ~ the arm} 57 are switches 58 and 59 that are engaged by the arm 57 when the telescope 55 is swung through as small an arc as a fraction of a degree 
On { HEAD ~ either side } of { TAIL ~ the arm } 57 are switches 58 and 59 that are engaged by the arm 57 when the telescope 55 is swung through as small an arc as a fraction of a degree
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH