1883.] 



the Dark Glasses and Mirrors of Sextants. 



43 



that their axes may be in the same horizontal plane. The eye-piece 

 •of the telescope, E, carries a parallel wire micrometer, F. Gr is the 

 dark glass to be examined, and H is another glass of the same tint. 



In order to adjust the instrument, the telescope, A, is directed to 

 the sun, a shade being fitted to the eye-piece, and then placed in its 

 Y's focussed for parallel rays. The collimators, B and C, are then 

 fixed on their table with their object-glasses opposed to that of the 

 telescope, A, the eye-pieces and wires having first been removed, 

 and a metal plate with a sharply cat hole in its centre, fitted to their 

 diaphragms. 



Light is next reflected down the collimator by the mirror D, and 

 the aperture in the diaphragm, being viewed through the telescope A, 

 is carefully focussed by moviug the object-glass of the collimator to 

 and fro, by means of its rack and pinion. 



The diaphragm aperture is next collimated by rotating the collimator 

 in its bearings. 



Both collimators being thus adjusted they are placed side by side, 

 so that their illuminated sights can be viewed simultaneously in the 

 telescope, appearing as superimposed bright disks 12' in diameter. 

 They are next separated so that the disks remain merely in contact at 

 the extremity of their horizontal diameters. 



The instrument is now ready for use, and the examination of the 

 shades is performed in the following manner. 



The glass to be tested is fixed in a holder, in front of the 

 •object-glass of collimator B, a corresponding shade being placed 

 between the heliostat and diaphragm of collimator C. The sun is 

 directed on to the diaphragms. The coloured disks are viewed 

 through the telescope, when if the sides of the shade G are per- 

 fectly parallel the relative position of the disks is unchanged, if, how- 

 ever, the shade is not ground true, the disks will appear either 

 separated or to overlap. In the first case, the amount of separation 

 is measured by the micrometer, F, and serves to indicate the 

 quality of the glass. In the case of overlapping images the shade is 

 rotated through 180°, and separation produced which can be measured. 

 A second examination is then made , the shade having been turned 

 through 90°. 



If in no position a separation of images is found to exist to the 

 extent of 20", the glass is etched K.O. 1 ; if more than 20" but less 

 than 40'', the mark is K.O. 2, with greater distortion than this, the 

 shade is rejected and not marked. 



To examine the quality of the mirrors, a small table, on levelling 

 screws, is put in front of the object-glass of the telescope. The 

 mirror to be tested is placed on its edge on this table, and turned 

 until a distant well-defined object is reflected down the tube of the 

 telescope. The object-glass of the telescope having previously been 



