focusing the telescope. No matter how well fitted the objective or 

 ocular draw tube may be when they leave the manufacturers hands 

 the inevitable wear soon permits lateral displacement with its well 

 known effect on the collimation. We were therefore forced to 

 abandon both of the older types of focusing which required move- 

 ment of either the objective or of the cross wires, and to seek some 

 means of focusing which would permit these two elements to re- 

 main in unalterable relationship with respect to each other. 



Fig. 43 



This condition required the interposition of some moveable 

 element between the objective and cross wires and at first glance it 

 might seem that lateral displacement could not be avoided here any 

 more than in the older types of focusing adjustment. In fact it 

 cannot although it can be reduced in amount and the feature of 

 most importance, the effect of a given amount of displacement, has 

 much less effect on collimation than an equal displacement of 

 objective or cross wires. 



We found that the conditions were best fullfilled by employing 

 a negative lens of low power as the focusing element. The manner 

 of its working is shown in Fig. 43. The cross* wire diaphragm at 

 F' is placed beyond the principal focus of the objective at F. The 

 plane of the cross wires is then conjugate to an object plane at a 

 short distance ahead of the objective and 

 if the negative lens is brought into the 

 plane of the cross wires so that its effect 

 is zero, that near object will be in focus. 

 An object at a great distance will be 

 imaged by the objective in its focal plane 

 at F, but by shifting the negative lens 

 towards the objective a point will be 

 reached where its dispersive action is 

 sufficient to prevent the formation of an 

 image ahead of F' and therefore this image 

 also lies in the plane of the cross wires. 

 Between these limits an object at any 



distance may be focused on the plane of the cross wires by moving 

 the negative lens to the necessary position. 



Pi*. 44 



84 



