| Arc scale |
Indicates the number of degrees of an angle. |
Index arm |
Pivots at one end to allow the attached index mirror to reflect an object onto the horizon glass and swings along the arc scale on the other end to indicate what the angle measures. |
| Micrometer drum |
Rotates to make fine adjustments when measuring angles and indicates minutes of a degree of angle. It is attached to the lower end of the index arm. One complete rotation moves the index arm 1° along the arc scale. The drum has 60 graduations, each representing 1' of arc. |
| Vernier scale |
Indicates tenths of a degree of angle. It is attached on the index arm adjacent to the micrometer drum and has 10 graduations, each representing 0.1' of arc. |
| Index mirrior |
Reflects objects onto the horizon glass. |
| Horizon glass |
Allows the observer to view one object directly on one side while observing a second object reflected next to it. The half of the horizon glass next to the frame is silvered to make that portion of the glass a mirror; the other half is clear glass. |
| Telescope |
Directs the line of sight of the observer to the horizon glass and magnifies the objects observed. |
| Filters |
Protects the observer's eyes when viewing the Sun. |
| Release levers |
Disengages the index arm from the arc scale to allow the index arm to move freely. |
