The original description in 1905 by Emil Hertel of a device to measure proptosis had only two mirrors; it was actually never made because it was difficult to see the scales which measure the position of the cornea. (Strabismus 2008; 16:45)
Two additional mirrors were added by the manufacturer, Carl Zeiss, to help superimpose the image of the rulers, which are at the sides of the frames, onto the image of the cornea.
The position of this second set of mirrors dictates whether the patient needs to fixate with the eye being measured. If the second set of mirrors are at the sides of the frame the image of the cornea and the image of the ruler are in the same plane.
Hertel's device was designed to replace the Birch-Hirschfeld exophthalmometer, described in 1900, which was more expensive and more difficult to manipulate, which in turn replaced the Weiss exophthalmometer, described in the 1890s, which was bulky and cumbersome.
Descriptions of various exophthalmometers date back at least to the mid-1800s.
The Naugle exophthalmometer was introduced in the early 1990's as an alternative to the Hertel exophthalmometer (Ophthalmic Surg 1992; 23:836).
Naugle's device is similar in design but it uses the superior and inferior orbital rim as a baseline
There are numerous circumstances, including trauma, tumor and surgery, in which the lateral orbital rim has been displaced and can no longer be used as a reference point.