Mesoamerican Long Count calendar 18. Thus 0.0.1.0.0 does not represent 400 days, but rather only 360 days and 0.0.0.17.19 represents 359 days. The name "b'ak'tun" was invented by modern scholars. The numbered Long Count was no longer in use by the time the Spanish arrived in the Yucatán Peninsula, although unnumbered k'atuns and tuns were still in use. Instead the Maya were using an abbreviated Short Count. Long Count dates are written with Mesoamerican numerals, as shown on this table. A dot represents "1" while a bar equals "5". The shell glyph was used to represent the zero concept. The Long Count calendar required
Fixation disparity A patient's associated phoria is the amount of prism needed to reduce their fixation disparity to zero minutes of arc. The Mallett Fixation Disparity Unit Instrument used to measure the associated heterophoria (or compensating prism). It consists of a small central fixation letter X surrounded by two letters O, one on each side of X, the three letters being seen binocularly, and two coloured polarized vertical bars in line with the centre of the X which are seen by each eye separately. The instrument can be swung through 90° to measure any vertical fixation disparity. The associated phoria is indicated