LEVELING 81 



GENERAL PROPERTIES OF LEVELS 



Sensitiveness of Level Bubble. The sensitiveness or delicacy 

 of the level bubble is indicated by the angle through which the 

 line of sight must move in order to cause the bubble to move 

 over one division of it. The smaller the angle, the more sen- 

 sitive will be the bubble. The tangent of this angle can be 

 determined by setting up the instrument and taking two rod 

 readings at a distance of, say, 400 ft. from the station. Take 

 one reading when the center of the bubble is exactly at a 

 division mark of its scale, and then by means of the leveling 

 screws, tip the instrument just sufficiently to cause the bubble 

 to move one division of its scale and note again the reading of 

 the rod. If the difference of the two rod readings is r, the 

 distance of the rod from the station is d, and the angle through 

 which the line of sight has been moved is a, then 



r 



tan a = - 

 d 



Magnifying Power and Definition. The magnifying power 

 of a telescope is the measure of its capacity to enlarge the 

 apparent size of an object. It is commonly expressed by the 

 number of times greater any linear dimension of an object 

 appears when viewed through the telescope than when viewed 

 with the naked eye, and is commonly spoken of as the number 

 of diameters of magnifying power. 



The magnifying power of a telescope can be determined 

 approximately in the following manner: Cut out a white card 

 exactly .1 ft. in width and attach it to a leveling rod so as to 

 cover exactly one of the tenth divisions; set up the rod at a 

 distance of, say, 25 ft., direct the telescope toward the rod, and 

 focus it perfectly. Then, by observing the rod with both eyes, 

 but with one eye looking through the telescope, note the num- 

 ber of divisions on the rod, as viewed with the naked eye, that 

 appear to be covered by the white card, as viewed through the 

 telescope. This will be, approximately, the number of diam- 

 eters of magnifying power of the telescope. It is well to 

 repeat the observation with the other eye looking through the 

 telescope. 



The definition of a telescope indicates the degree of clearness 

 and sharpness of outline with which objects can be seen through 



