sectional view of section 20c taken along line 7 7 of fig6 section 20c is substantially shaped like a cylinder and it is tightly surrounded by cylindrical portion 29a of cover 23 rods 22a 22c slide through holes 40a 40c respectively to support section 20c set screws 42a and 42b firmly hold section 20c to the two outer rods 22a 22b another pair of set screws not shown is positioned at the other end of section 20c also for firmly holding that section to the two outer rods 22a 22b section cavity 44 extends through section 20c optical components such as prism 4 not shown are mounted in a subholder not shown which is inserted into section cavity 44 and fastened in place with three set screws 45a 45b and 45c beam 9 will pass through section cavity 44 and any optical component positioned there fig8 is a cross sectional view of sensor chamber 21 taken along line 8 8 of fig6 sensor chamber 21 has three holes 50a 50c which only partially extend into the sensor chamber rods 22a 22c are mounted in each hole 50a 50c where they are held in place by set screws 52a 52c respectively opening 54 provides the entrance for laser beam 9 into sensor cavity.