Even though the theodolite was designed around 1556, it took three hundred years to really catch on with surveyors. It is made with a flat horizontal disk like the astrolabe that is divided into 360 degrees, with or without a compass mounted in the middle, and a circular or half circle plate or ring oriented vertically or at right angles to the horizontal dial and graduated over 90 degrees for measuring vertical angles. This two dimensional measuring capability is called altazimuth, the instrument more commonly called, ‘two-circle’. Theodolites developed into many different models, designs and sizes for specialized applications and diverse environments that can vary from high and airy mountain peaks to deep and cramped underground mines. For example, mine theodolites typically have either pivoting auxiliary telescopes mounted atop the vertical circle or the primary scope to allow sighting vertically up or down a shaft or winze or orient to nadir, or a hollow base axis so the surveyor can sight vertically down. In the early 1900s makers began enclosing the circles on theodolites for protection in the field and mines, and in the late 1920s added interior optics with protruding magnifying scopes to read the dials. Compact theodolites with a half-arc vertical dial that is breached at the top and bottom to create two arcs are called, ‘Mount Everest type’. Micrometer theodolites that allow greater precision became available early in the 1900s. Theodolites with optical distance measuring capabilitiea are called tacheometers. The gradioplane is a specialized theodolite that appeared about 1910. It has a normal horizontal dial and compass but in place of a vertical dial it sports a large helical ‘gradienter screw’ that allows measurement of very small vertical angles. Transit theodolites are designed with telescopes that swing in the vertical plane forward and backward 180 degrees in order to back sight. (More about transit levels below.)