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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



X-ray Measurement of Axial Length of the Eye 



A sheet of X-rays is produced by passing X-rays 

 through two parallel slits, and the eye is held so that 

 this sheet of rays traverses the eye perpendicular to 

 the direction of regard, as shown in figure i6. Its 

 intersection with the retina is a circle and, since 

 X-rays so applied stimulate the retina, the subject 

 sees a circle of light. The size of the circle can be made 

 smaller and smaller by moving the sheet of rays 

 toward the back of the eye, keeping it always per- 

 pendicular to the direction of regard. It reduces to a 

 point as it becomes tangent to the retina. The distance 

 of the cornea from this plane can then be measured by 

 sighting on the profile of the cornea (23). (Care 

 must be taken to keep the X-rays from passing through 

 the crystalline lens.) 



X-ray Determination oj Location of Second .Nodal Point 



Two sheets of X-rays are produced by passing 

 X-rays from a source through one slit and these then 

 are passed again through two slits which are parallel 

 to the first slit as shown in figure 17. These two sheets 

 of rays are allowed to traverse the back of the eye 

 which is pointed with its optic axis -in a direction 

 parallel to the slits. Two distant object points are 

 adjusted so that their images fall on the two lines 

 on the retina stimulated by the X-rays. Knowing the 

 linear separation of the two X-ray images and 

 knowing the angular separation of the two optical 

 images at the second nodal point, one can compute 

 the distance from the second nodal point to the retina. 



Locating Conjugate Focus of Retina 



It is assumed in this section that the astigmatism 

 of the eye, if any be present, has been corrected and 

 that the experimenter is interested only in locating 

 the conjugate focus of the retina for the purpose of 

 determining the refractive state of the eye or the 

 amount of accommodation in play. In order to 

 simplify our problem let us consider absolute pres- 

 byopia in which the eye has a fixed focus. The same 

 thing occurs in an aphakic eye or in an eye which has 

 been temporarily paralyzed with a drug. In this 

 case a target is needed to control fixation and then 

 some subjective or objectiv'e means must be provided 

 for locating the conjugate focus of the retina. 



Let us consider the subjective methods first. A 

 target consisting of a point, a line or a row of small 

 letters can be used for controlling fixation and lo- 

 cating the conjugate focus of the retina. The distance 

 can be varied or lenses may be placed in front of the 

 eyes. The natural pupil may be used or, if this has 

 been dilated, a diaphragm with a pupil of normal 

 size can be placed before the eye. The target must 

 be fine enough so that the subject can tell when 

 the focus is sharpest. 



The Scheiner principle substitutes for the natural 

 pupil two small holes or two parallel slits. If a single 

 target is used the retinal image doubles when it is 

 out of focus (fig. 18). The doubling is easier to detect 

 when monochromatic light is used than when white 

 light is used; still better the two beams can be trans- 

 mitted through filters of different color. If the upper 

 and lo\ver halves of a vertical slit are seen through 

 different parts of the pupil displaced laterally from 



Fig 16 



Fig^l7 oPTK 



FIG. 16. X-ray measurement o( the axial length of the eye. fig. 17. X-ray method of locating the second nodal point. 



