76 



ing or astronomical telescopes, unless during an observation the aperture of the 

 objective is cut down to i inch diameter, by means of a diaphragm placed in front of 

 it, when the image can be seen as sharply defined as those of the erecting telescope ; 

 or the observations must be made with the shade glasses and reflecting prism alone. 



7. Using the Colored Shade Glass. 



a. Directions. Attach the colored glass shown in fig. 4, to the eye-piece, 

 to shield the eye from the sun and look directly at it, setting the cross-wires so as 

 to quarter it. 



b. Remarks. This will be found entirely satisfactory when the sun's alti- 

 tude is so low as to enable the observer to bring his eye in apposition with the eye- 

 piece of the telescope with ease. 



8. Using- the Diagonal Eye-piece. 



a. Directions. Screw on the prism, as shown in fig. 3, to the end of the 

 common eye-piece. Look directly through the shade-glass, if observing in that 

 way, turning the prism either way so as to make it convenient to look into it. If 

 any trouble is experienced in finding the sun with it. let the sun first shine through 

 the telescope, the colored shade-glass being turned aside, till the brill'mnt. light 

 perceived in the aperture of this eye-piece shows the telescope to be rightly 

 directed. Cover the aperture with its shade-glass and proceed. 



b. Remarks. By attaching the reflecting prism to the eye-piece of the tele- 

 scope, the light is reflected at right angles to the the line of sight of the telescope, 

 and it thus becomes what is termed a diagonal eye-piece. 



This prism can be used for direct observation when the altitude of the sun is 

 too great to allow the eye to be applied directly to the eye-piece of the telescope, 

 and not so great as to bring the eye-piece too far over the plate, but through t his 

 range of altitudes the solar screen can be used without the prism, as shown in fig. 2, 

 and it will usually be found advantageous to do so. 



Since the prism in effect withdraws the eye about half an inch further from the 

 eye-piece of the telescope than its natural position, that being about the distance 

 traversed by the light in passing through the prism, the high magnifying power 

 used in C. L. Berger & Sons' transit telescopes makes the use of the reflecting 

 prism for direct observation a little awkward, and it will usually be found more 

 satisfactory when using the prism to use the solar screen with it. 



9. Using the Reflecting Prism and Solar Screen combined. 



a. Directions. Attach the prism, and direct the telescope as in 8. Then, 

 leaving the aperture of the prism uncovered, adjust the solar screen so as to receive 

 the images of the sun and the cross-wires, as shown in fig. 1. 



b. Remarks. For observing the sun at high altitudes it will be found that in 

 this, otherwise most difficult of all positions, the use of the solar screen combined 

 with the prism will enable the engineer to make his observation with the greatest 

 ease and precision. 



ID. Making the Observations. 



a. Directions. Direct the telescope to the sun, and by means of the slow 

 motion screws, cause the image of the cross-wires to exactly quarter the sun's im.i^e. 

 Read both circles and record the readings. Refer the position of 1 he inst rument to 

 some fixed line, and once, after the above work, by another plate reading. A!-o 

 note and record the exact instant of time of the observation by the watch. 



b. Remarks. This observation with the watch may be used as hereafter 

 indicated to simplify and lessen the amount of work in making the reductions. A 

 fair watch of ordinary accuracy is sufficient. The entire work can be carried on 

 without a watch at all, but it takes some more figuring. 



11. Use of the Nautical Almanac. 



a. Remarks. In order to use the observations, made as above directed, it 

 is necessary to find the sun's apparent declination for the time of observation. 

 This is done as directed below. 



6. Conditions. Let all the algebraic signs be carefully observed throughout 

 the work. Use the watch time. 



