(see top p. 103), and where Ver. B. is consulted, the effort is no 

 greater than when opposite verniers where placed between the legs 

 of the standard. 



These instruments are designed so that the telescope swings 

 through the standards at theobjective end without special preparation 

 or precaution (seep. 189) and, while the sun shade may occasionally 

 touch the frame of the vernier reflector, there will be no danger of 

 damage incident to the contact. Special provision in the way of 

 buffers are not necssary w ith these new telescopes. 



Occasionally for mining purposes we have placed the vernier 

 windows immediately under the line of the telescope. In cramped 

 positions, this arrangement is very convenient. 



The Edge Graduated Vertical Circle, for the same reason, is 

 especially important. In boggy ground or in other precarious positions 

 the instrument that permits readings to be taken without moving in 

 one's tracks offers special advantages that will be appreciated by 

 those who have experienced the discomforts and irritation of an 

 inaccessible vernier opening. 



The Lens System 



The interior focusing system, peculiar to the Tachymeters and 

 Theodolites, and the variable power eyepiece, applicable to all 

 classes of instruments, are distinctive features which merit detailed 

 description and explanation. Before entering upon their descrip- 

 tion, however, we present a brief exposition of the theory of the 

 telescope which we believe will be of interest to every engineer. 



The Function of the Telescope is twofold: first, it serves 

 to bring the object into the plane of the cross wires without parallax 

 and second, it serves to make visible objects which would be in- 

 visible to the naked eye. The extent to which the first of these 

 is accomplished depends entirely upon the accuracy with which the 

 telescope is focused. The performance of the telescope with 

 respect to the second consideration depends upon the resolving 

 power of the objective, magnification, brightness of image, and the 

 perfection attained in the design and manufacture of the lenses. 



The Resolving Power of an objective is the measure of its 

 ability to form separate and distinct images of two neighboring 

 points of the object. Its value depends upon the aperture of the 

 objective and is expressed by the angle subtended at the center of 

 the lens by the distance separating the two closest points which it 

 can image separately, i. e. , resolve. The relation connecting resolv- 

 ing power and aperture is -vy" wherein is the resolving 



power in seconds of arc, and D is the aperture expressed in centi- 

 meters. It applies without modification only to white points on a 

 black back ground. An objective of one inch aperture will then 



