The negative lens mast, of course, move back and forth with a 

 minimum of lateral displacement. To this end the focusing lens 

 barrel is guided by three longitudinal ribs integral with the tele- 

 scope tube and turned to a center in common w ith the objective 

 mount. Fig. 44 shows a cross section of the tube. This arrang- 

 ment provides an accurate seating for the focusing-lens barrel 

 throughout its entire range. 



Assuming, however, that slight lateral displacements occur we 

 can demonstrate that the effect on collimation is much less than the 

 effect of an equal displacement of the objective. Referring to Fig. 

 45, let O represent the objective and R the diaphragm. Let N be 

 the position of the negative lens when the collimation line, AB, 

 was established and assume that in focusing on a nearby object the 

 negative lens moves out of alignment to the position Ni. F is the 



* 



Fi*. 45 



anterior focal point of the negative lens and f its focal length. The 

 axial ray through the center of the objective, O, will be deviated by 

 the focusing lens as shown in the diagram and produce a deflection 

 x at the diaphragm. If 3 represents the lateral displacement of the 

 lens we have: x : / : : y : J 



_ jry 



In our 5" Erecting Tachymeter f2S5 mm and _y=92 mm 

 when focused on a point 5 ft. ahead of the objective. Let us 

 assume / = 0.02 mm, then 



jr= ' 02 * 92 .= 0.0064 mm or about ^ / 



In our 5" Inverting Tachymeter /^448 mm and y -148 mm 

 when focused for 10 ft. Substituting as before, 



x = -^ 4i y-= 0.0065 or again 'A > 



Whatever the lateral displacement of the focusing lens the 

 effect upon collimation does not amount to more than the third part 

 of it. In the older types of focusing any lateral displacement has 

 its full effect upon collimation. 



In addition, the interior focusing system permits more nearly 

 water and dust proof construction as well as less disturbance of 

 balance in the process of focusing. 



85 



