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ABBE APERTOMETER

MAKER: ZEISS

c. EARLY 20th C.

abbe apertometer


The Abbe Apertometer, the first commercial type to come into common use, was described in an article in the JRMS of 1877-8. The image above shows the Abbe Apertometer in use. A special low power objective(not visible in this image) is in place at the bottom of the drawtube, which sits inside the main optical tube. In use with a traditional configuration, light is directed into the horizontal opening of the device. The objective at the end of the drawtube turns the drawtube itself into a microscope to view the back focal plane of the objective as the NA is determined. The Abbe apertometer, was for many years, the most common form, though its design changed slightly. Although many other types of apertometer were developed, the Abbe is by far the most common form still extant today, and also the easiest to use and probably the most precise, particularly for high NA objectives. Among the other forms developed was the Cheshire Apertometer, and the Beck Apertometer. An example of the Beck apertometer is in this collection.

The author is very greatful to James Solliday for sharing his image of the Abbe apertometer in position on a microscope.


HISTORY OF THE ABBE APERTOMETER
Ernst Abbe, originator of the numerical aperture concept, made the first instrument called an apertometer in 1871 and his first example is part of the London Science Museum collection. That instrument was rectangular with a graduated brass plate on top. It had a prismatic piece of glass glued on to its long side. It is Bracegirdle's 39/38, inventory 1928-899 and found in section 'F' of his CD. According to Zeiss, Abbe first introduced the Numerical aperture term (NA) in 1873, and the first apertometers of the type shown here were designed by Abbe about 1876, and Zeiss was apparently selling them shortly thereafter. The first JRMS article about this type of apertometer was published in 1878.

Initially the glass of the apparatus was made of crown glass, but by 1881, this was changed to flint glass which, having a higher refractive index, required a shorter distance between the indicators, which made it easier to use. In the JRMS of 1881, Mayall Jr. quoted the flint glass refractive index at 1.631. In reality, one modern source quotes the refractive indices for Crown glass to range from 1.52 to 1.62, and that of Flint glass to range from 1.57 to 1.75. In fact, the glass in each Zeiss Abbe refractometer has a slightly different refractive index and each is actually engraved with the value for that particular piece of glass. This was neccesary to correctly space the calibration lines. A sampling of actual labeled values taken directly from Zeiss Abbe refractomers is: 1.6214, 1.6233, 1.62507, and 1.62549. The illustration in the 1924 brochure and instructions for the Abbe apertometer shows an example with a refractive index of 1.62535. It would seem that most of the later examples have a refractive index over 1.62 and less than 1.63. The refractive index of the glass being used is neccesary to correctly calibrate the engraved scale on each apertometer. The distance between the values on the scale will be shorter with higher NA glass.

early abbe apertometer later abbe apertometer

The earlier commercial versions of the Abbe Apertometer had two moveable indicators(left), but later, as shown to the right, and at the top of the page, a single cursor was used in the later version. In the earlier type a metal pointer on the indicator on each side was used to determine the NA, but this was later changed to a single line cursor. I could find the cursor type in references starting in 1924. The double indicator type was still offered in the Zeiss catalog of 1913; therefore the change occurred sometime between 1913 and 1924. Unfortunately I have been unable to locate any dated catalog containing an entry for the apertometer after 1913 and before 1924 to further narrow down the search. The 1920 edition of Spitta still showed the older version, so it is reasonable to conclude the new model with a single cursor was first produced in the 1920's. I would greatly appreciate information from anyone with catalog entries for the Abbe apertometer after 1913 and before 1924 to further narrow this down.

For many years two versions of the Abbe apertometer were offered. One simply supplied the glass and indicators or cursor. In those models, these parts were simply pushed along against the glass, not held in place by anything except ones fingers. The more deluxe models had a metal base added for these parts to ride in a groove for smoother operation.

For the interest of the reader, Zeiss catalog entries of the Abbe apertometer are provided for 1889. 1913, and 1934. You can also see the illustration, from the instruction sheet '114e' from 1924.

For a discussion of the general history of the apertometer, and for other types of apertometer, please see the apertometer history page.

The author is indebted to James Solliday, president of the Microscopical Society of Southern California for help with some of the information and several illustrations on this page.

USING THE ABBE APERTOMETER
Using the Abbe apertometer to determine NA is usually reasonably accurate, provided instructions for its use are carefully followed. The instructions by Zeiss leave out a few helpful details on improving accuracy mentioned by Spitta and in greater detail by Hartridge. Hartridge goes further to make several suggestions for improving accuracy employing a reversed light path, that is with the light directed down the optical tube and the NA determined by the pattern of light falling on the apertometer. In addition, several other caveats about Hartridge's method make his method quite tedious. That degree of accuracy, though it may be improved with his method, is not neccesary for ordinary work, as the errors occurring when the device is employed when the device is used as originally intended, are too small to be detected by the human eye. That is, the human eye cannot differentiate a change in resolution with small changes in NA. A satisfactory result can be obtained by combining the instructions by Zeiss and the precautions mentioned by Spitta.

Outline of use:

  1. Place the instrument on the stage
  2. Center the little coverslip under the objective
  3. With a bright light source aimed a the Apertometer's round side, more precisely center the line between the two hemicircles of silver under the coverslip and focus the edge of these using the usual method
  4. Taking care not to disturb the focus, attach the special objective to the inside distal end of the drawtube and slide it in or out to focus on the cross of the cursor. Alternatively, use a phase telescope to do the same. This will obviate the need for using the auxilliary objective.
  5. With a bright light shining through the aperture of the cursor, using the drawtube or phase telescope, or a Bertrand lens,focus on the cross.
  6. Move the cursor to one side and note the reading where the center of the X reaches the edge of the field; do the same in the other direction
  7. The NA is the average of the two readings
  8. Note that the above can be performed with the naked eye alone with the eyepiece removed for the lower power objectives. An auxilliary magnification system is not needed for low power objectives.
  9. Note that immersion oil must be used for oil immersion objectives.
  10. Note that with the older model with two indicators, one has to focus on the edge of the indicator as opposed to the cross in the cursor.
  11. REFERENCES

    1. Carl Zeiss (company) (1924): The Abbe Apertometer Leaflet Mikro 114 (2nd ed).
    2. Gunther, N (1951) Ernst Abbe: creator of the Zeiss Foundation. Transl. by David Bower (2016)
    3. Hartridge H (1918) An Improved Method of ApertometryJRMS pp337-348.
    4. Ingpen, John (1878) Description of Professor Abbe's Apertometer with Instructions for its use, By Carl Zeiss of Jena. JRMS Vol I pp18-22
    5. Mayall Jr, J (1881)JRMS Ser II, Vol I, Part 2 p978
    6. Spitta, Edmund J: (1920) Microscopy. The Construction, Theory and Use of the Microscope. 3rd ed. pp94-98.
    7.