52 



The Wye Level. 



The description of the telescope of the engineer's transit applies with the fol- 

 lowing modilications to the telescope of this level. 



It has a clear aperture of 1^ inches focus, and is 17 or 18 inches long over all, 

 the sun-shade excluded. 



The bell-metal collars which rest in the wyes are about 10^ inches apart and 

 \Y inches in diameter. 



On account of the extreme length of the telescope tube, four capstan-headed 

 screws are provided for centering the eve-piece. 



The object-glass focussing screw is in the middle of the tube. The eye-piece Is 

 focussed by turning a milled ring at the eye-end. The level attached to the tele- 

 scope is about 8 inches long, with about 5 ^ inches exposed, over which is placed 

 the metal scale for reading the position of its bubble. The level-tube is suspended 

 from the telescope-tube in such a manner that at the object-glass end it can be moved 

 in azimuth, with reference to the telescope axis, and at the eye-piece end it can be 

 moved in altitude with reference to the same axis. 



Its graduated scale has its graduations set carefully opposite each other on its 

 two sides, and they are numbered from 5 to to 5 at each end of the bubble. 



'Since it is not necessary to construct a level which shall have absolutely the 

 game value in arc for the same motion of its bubble throughout its length for 

 engineering purposes, the graduated scale is so set that the slight deviations from 

 the arc of a circle may be equally distributed on each side of the zero of the scale. 

 The bubble tube is ground cylindrical. 



The level-bar is about 12 inches long over all, and at its two extremities sup- 

 ports the two wyes which rise about 3^ inches from its upper surface. One of 

 these wyes is adjustable in altitude. The level-bar is attached to a long conical 

 center of the hardest bell-metal, which mav be clamped to the upper level plate, and 

 then a slow motion In azimuth may be given to the telescope, by a slow motion 

 screw which presses the clamping bar against a stiff spiral spring. With the sun- 

 shade on the telescope, the weight is equaly distributed from the center, each way. 

 This is necessary, since a sensitive level, in the nicest work, is affected by any 

 nnequal strain, though it may seem to be, practically, imperceptible. 



The base, on which the leveling screws rest, has as groat a diameter as porta- 

 bility will permit; and the leveling screws are cut with a flue thread. These two 

 points add to the ease with which the instrument may be accurately leveled. 



A stop is so arranged that the telescope may be readily set with its horizontal 

 cross-line level, when the instrument is in adjustment. 



The instrument complete is not separable when put into its box. Messrs. C. L. 

 Berger <fc Sons, believe this condition to be necessary to protect one of the essential 

 adjustments of the level the adjustment of the wyes from needless derangement. 



This instrument is sometimes made by Messrs. C. L. Berger &. Sons in a differ- 

 ent form. One of the wyes is movable in a vertical line by a milled-head screw. 

 This enables pointing to be made with greater accuracy aiid facility. 



The Adjustments. 



In a theoretically perfect level the following points are established : 



1. The object and eye-glasses are perpendicular to the optical axis at all distances 

 apart. 



2. The optical axis coincides with the axis of rotation in the wyes. 



3. The axis of collimutioii coincides with the optical axis. 



4. The axis of collimat ion is parallel to the telescope level. 



6. The collars resting in the wyes are circles of the same diameter and concen- 

 tric with the line of collimation of the telescope. 



6. The wyes are exactly similar, and similarly placed with reference to tin- line 

 of collimatiou of the telescope. 



7. The l.-vel bubble moves over equal spaces for equal displacements of 1 lie 

 telescope in altitude. 



8. The level bubble expands or contracts equally from the center in both 

 direction-, during changes of temperature. 



9. k:al axis of revolution is perpendicular to the line of collimation of 

 pe. 



Of the above, the maker establishes points numbered 1, 2, 5, 7 and 8. The 

 reiiMiiiiiii; points, 3, 4 and 1. are ->taMMie| when the instrument leaves the shop. 

 but being liable to derangement from rough usage, they are made adjustable in the 

 field. 



