FijJ - 



Diaphragms 



Theodolite Diaphragms, used for geodetic work or field astron- 

 omy are touched upon on p. 108. No. 9 is used for a combination 

 of stadia and solar work, but the cut does not show a strict propor- 

 tion of spacing. The stadia interval is 34' 23", while the average 

 diameter of the sun, measured in the celestial sphere, is only a trifle 

 over 32'. The solar square, however, is purposely made smaller in 

 order to facilitate centering by dividing the sun's image into equal 

 segments. Cuts 10, 11 and 12 are used for the observation of time 

 stars. 



In geodetic operations the style of signal used depends on the 

 distance, the atmospheric conditions and the resolving power of the 

 objective. If the vertical wires are twin lines, spaced 20" apart, as 

 shown in Fig. 80, they will intercept an interval 

 of about 6 in. per mile. If signals are built in 

 this proportion, centering can frequently be 

 accomplished more accurately than by trying 

 to bisect with one w r ire. The same idea is 

 carried out in the filar micrometer eyepiece of 

 the reading microscope considered on p. 49. 



Glass Diaphragms have been used more 

 or less for a great many years on account of their permanency; but 

 in quickly changing temperatures the surfaces will fog from the 

 condensation of what moisture may be inside the tube. The new 

 cryptic focus, described on p. 83, has nearly, if not quite, over- 

 come this tendency and we have used glass diaphragms more 

 recently with success. The lines are cut with a diamond point and 

 do not need a pigment filling. Such lines, now employed for our 

 Compensation Level (see p. 29), when carefully focused are very 

 sharp and distinct. In Germany they have succeeded in photo- 

 graphing the lines on the glass of late years with an emulsion of 

 extremely fine grain and dense texture. It is said that the back 

 side of this microscopic line can be made white to facilitate illumin- 

 ation for night work. 



Glass diaphragms, when supplied with our Theodolites and 

 Tachymeters, are not to be feared as previously. With the 4)4 -inch 

 instrument the diaphragm is readily accessible in the possible event 

 of fogging, in both the erecting and inverting telescope. If the 

 ocular mount in either case is unscrewed, the diaphragm will be 

 found to lie immediately underneath. If the dew is on the ocular 

 side, it may be removed with a clean linen handkerchief. If on the 

 objective side, remove the top and bottom diaphragm screws and 

 tip the reticle over, using the side screws as gimbals. After clean- 

 ing, replace and readjust for collimation. 



With the 5-inch inverting telescope the process is equally 

 simple, but with the 5-inch erecting telescope the whole eyepiece 



142 



