84 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



head. Q, on the side of the telescope, that beir.g the means 

 of adjusting- it to show an object distinctly. 



A brass plummet and line are packed in the box with the 

 theodolite, to suspend from a hook under its centre, by 

 which it can be placed exactly over the station from whence 

 the observations are to be taken ; likewise, if required, two 

 extra eye-pieces for the telescope, to be used for astrono- 

 mical observations ; the one inverts the object, and has a 

 greater magnifying power, but having fewer glasses pos- 

 sesses more light ; the other is a diagonal eye-piece, which 

 will be found extremely convenient when observing an 

 object that has a considerable altitude ; the observer avoid- 

 ing the unpleasant and painful position he must assume in 

 order to look through the telescope when either of the 

 othei eye-pieces is applied. A small cap, containing a 

 dark-colored glass, is made to apply to the eye-end of the 

 telescope, to screen the eye of the observer from the inten- 

 sity of thtf sun's rays, when that is the object under observ- 

 ation. A magnifying glass, mounted in a horn frame, a 

 screw-driver, and a pin to turn the capstan-screws for the 

 adjustments, are also furnished with the instrument. 



The V tamer. This is a contrivance for measuring parts 

 of the space between the equidistant divisions of a graduat- 

 ed scale. It is a scale whose length is equal to a certain 

 number of parts of that to be subdivided, depending on 

 the degree of minuteness to which the subdivision is intend- 

 ed to be carried ; but it is divided into parts which in number 

 are one more or one less than those of the primary scale 

 taken for the length of the vernier : in modern practice, the 

 parts on the vernier are generally, one more than are con- 

 tained in the same space on the primary scale. 



If it is required to measure to hundreclths of an inch, the 

 parts of a scale which is graduated to lOths. it may be done 

 by means of a scale whose length is nine tenths of an inch, 

 and divided into 10 equal parts ; or by one whose length is 

 eleven tenths of an inch, and divided into 10 equal parts ; 

 for in either case the difference between the divisions of the 

 scale so made and those on the primary scale is the hun- 

 dredth of an inch. Such a scale, made to move along the 

 edge of that to be subdivided, is called a vernier. By its 

 application, either to straight lines or arcs of circles, the 

 subdivisions of graduated instruments are read off. 



The adjustments. The first adjustment is that of the line 



