312 Hastings — Error of Collimation in the Human Eye. 



sider later, is the question of the position of the pupil of the eye. 

 Here too, we lack sufficient data for establishing any conclusion. 

 As far as is known to me, only three eyes have been investi- 

 gated in this particular, and all of these by Helmholtz (H. p. 

 29) ; and as he found in two of the three that the center of the 

 pupil falls very near the axis of vision, and in the third very 

 near the axis of the eye, no result could be less determinate. 

 Were the base of the cornea centered upon its axis of rota- 

 tion as Helmholtz concluded from his limited number of obser- 

 vations (H. p. 19, relation of angles a and /3), the determination 

 of this element would be very easy ; indeed, a simple inspec- 

 tion shows that the pupils of practically all eyes, at least when 

 not dilated, are strongly displaced towards the nasal side of the 

 iris. But we found no such simple relation between the axes 

 of the cornea and of the iris ; hence it seemed necessary to 

 make a rather extended series of measures for a foundation 

 for precise conclusions. 



Helmholtz's method of determining this element by use of 

 his ophthalmometer is somewhat laborious and subject to con- 

 siderable uncertainties, if not to errors, from changes in pupilar 

 diameter during the measurements. This consideration, 

 together with the fact that statistical information founded 

 upon numerous observations of moderate accuracy is of far 

 greater value than an accurate determination of individual 

 peculiarities, led me to adopt a different method, which may 

 be described as follows. 



A telescope provided with a filar micrometer is placed at a 

 distance from the eye to be measured, which is large compared 

 to the linear dimensions of the eye. The telescope has directly 

 below its objective a small adjustable mirror wmich, illumin- 

 ated by a conveniently placed light, serves as a luminous source 

 itself. "When the observed eye is fixed upon the middle of the 

 objective of the telescope, the observer sees, with properly 

 adjusted telescope, a sharply defined image of the pupil with a 

 bright point image of the mirror near its center. Two parallel 

 wires of the micrometer are brought within a convenient small 

 distance of each other, and the telescope so directed that these 

 wires appear to be symmetrically placed upon the pupil ; then 

 the point image will, in general, appear to lie slightly on the 

 nasal side of a point midway between the wires. In a few 

 cases the bright point will appear exactly half way between the 

 wires, and even, in very exceptional cases, with a very slight 

 displacement towards the temporal side. 



Fig. 1 illustrates the general appearance when the nose lies 

 in the direction indicated by the arrow. When a satisfactory 

 adjustment of this kind is secured, an estimate is made of the 

 apparent displacement of the center of the pupil towards the 



