Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
Anatomical Description of the Eye 
[19, 20. 
goes through the hole and reaching the eye is reflected from the cornea. At 
F there is an adjustable mark which is used as the point of fixation of the eye. 
The ophthalmometer is placed first at Gi and then at G2, equidistant from B 
in both positions. Marks may be made on the table for the tripod of the 
ophthalmometer, as the adjustment of the telescope has to be changed during 
the experiment. The patient is told to look steadily at the fixation mark F, 
and to follow all its movements. The observer, making his first observation at 
Gi, adjusts the glass plates of the ophthalmometer until one of the double 
images of the luminous point in the cornea coincides with the edge of the 
pupil. If now the other double image does not also coincide with the other 
edge of the pupil, the marker F is moved along the scale hntil this occurs, 
and the scale division noted. This same procedure is repeated from the second 
position of the ophthalmometer at G2. 
The distance AB must be measured in divisions of the scale CD ; and then 
the angle FAB can be found by the formula: 
= tan Z FAB. 
Knowing the axis major AH of the corneal ellispsoid and the angle F AH, 
the angle BAH may be calculated. This angle is needed to find the position 
of the reflex image in the cornea. In the same way the angle GiAH is found, 
which establishes the direction along which the observer viewed the eye at A. 
The centre of the apparent pupil (as it looks through the cornea) will lie there¬ 
fore in a line parallel to GiA, which passes through the apparent position of 
the corneal image. 
The method by which the actual position of the centre of the pupil may 
be calculated from its apparent position will be given in §9 and §10.1 
The results of the measurements made with the ophthalmometer on the 
corneae of three eyes are as follows : 
0. H. 
B. P. 
J. H. 
Distance of the pupillary plane from the vertex 
j apparent 
of the cornea............................. 
( actual 
Distance of the centre of the pupil from the 
S apparent 
corneal axis on nasal side................... 
( actual 
That the iris lies in contact with the lens and is convex in front, has been 
a subject of much controversy among anatomists.2 The older anatomists 
assumed this to be true, until Petit denied it and asserted that the so-called 
;posterior chamber of the eye lies between the iris and the lens, as a result of his 
investigations on frozen eyes. In frozen eyes, thin sheets of ice are sometimes 
found between iris and lens. Almost all later anatomists accepted Petit’s 
view, until very lately Stellwag von Carion and Cramer again asserted 
the close apposition of iris to lens. The author has been able to make direct 
observations by the method mentioned above which appear to confirm this. 
Nevertheless, Budge (1855) has defended the work of Petit. 
1728. Petit in Mém. de l’Acad. Roy. des sciences. 1728. p. 206 and p. 289. 
1850. Stellwag von Carion in Zeitschrift d. Wiener Ärtze. 1850. Heft 3, S. 125. 
1852. Cramer in Tijdschrift der Nederl. Maatschappij tot bevord. der Geneeskunst. 1852. 
1 Helmholtz in Graefes Archiv, für Ophthalmologie. Bd. 1. Abt. 2, S. 31. 
2 1fSee the supplement to this section which quite adequately presents the present 
viewpoint. (D. H.)


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