418 SCIENTIFIC RECORD FOR 1883. 



For these purposes the following apparatus has been devised and 

 brought into use at the observatory: 



A telescope of 3£ inches aperture and 48 inches focal length, a pair of 

 collimeters of 1^ inches aperture and 1 inches focal length, and a heliostat 

 are firmly fixed to a stout plank, so that their axes may be in the same 

 horizontal plane. The eye- piece of the telescope carries a parallel wire 

 micrometer. 



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

 a shade being fitted to the eye-piece and then placed in its Y's focused 

 for parallel rays. The collimators are then fixed on their table with their 

 object-glasses opposed to that of the telescope, the eye-pieces and wires 

 having first been removed and a metal plate with a sharply cut hole in 

 its center fitted to their diaphragms. 



Light is next reflected down the collimator by the heliostat, and the 

 aperture in the diaphragm being viewed through the telescope is care- 

 fully focused by moving the object-glass of the collimator to and fro by 

 moans 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 sight can be viewed simultaneously in the tel- 

 escope, appearing as superimposed bright disks 12' in diameter. They 

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

 tremity 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 rotating frame in front of the ob- 

 ject-glass of one collimator, a corresponding shade being placed between 

 the heliostat and diaphragm of the other collimator. The Suu is now 

 directed on to the diaphragms. The colored disks are viewed through 

 the telescope, when, if the sides of the shade placed between the colli- 

 mator and the object-glass of the telescope are perfectly parallel, the 

 relative position of the disks is unchanged ; if, however, 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 microme- 

 ter, and serves to indicate the quality of the glass. In the case of over- 

 lapping images the shade is rotated through 180°, and separation pro- 

 duced 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 j if more than 20" but less than 40", 

 the mark is K. O. 2 ; with greater distortion than this, the shade is re- 

 jected and not marked. 



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

 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 dis- 



