THE IMAGE-FORMING MECHANISM OF THE EVE 



^59 



each other, the two halves of the slit are seen out of 

 vertical alignment when the eye is out of focus 

 (fig. 19). Monochromatic light is used in this case. 

 Since the Scheiner technique makes use of only two 

 small parts of the pupil, it may yield a different 

 measure of accommodation than a measurement 

 based upon the full pupil. This is a function of the 

 spherical aberration of the eye. Consequently the 

 Scheiner principle is primarily useful in measuring 

 changes in accommodation. 



The threshold principle involves having the target 

 (usually a line) disappear when it goes out of focus. 

 This test has the advantage that it can be used with a 

 normal pupil. 



The retinoscope (skiascope) is an objective device 

 for determining the refraction of the eye. A small 

 mirror throws a beam of light on the eye from a 

 small source. The examiner looks through a small 

 hole in the mirror and observes the light reflected 

 from the retina back out of the eye. This makes the 

 pupil appear bright, and moving the mirror modifies 

 the distribution of the light in the pupil so that the 

 examiner can tell when the eye is out of focus for the 

 hole in the mirror. When the eye has a fixed focus, 

 the subject can control his fixation by fixating the 

 mirror image of the light source. 



The coincidence optometer is another objective 

 device which is a very valuable means for the ob- 

 jective determination of the refractive state. In this 

 device an image is formed on the pigment epithelium 

 in contact with the retina, and the light difl'uscly 

 reflected from this surface forms an image in the 



plane conjugate to the retina. This image is viewed 

 through an eye piece. The target and the focal plane 

 of the eye piece are kept at the same distance from 

 the subject's eye, but this distance can be varied to 

 locate the conjugate focus of the retina. The operator 

 may use blur as a criterion for the proper setting, or as 

 in Fincham's instrument (24, 27) a modification of 

 Scheiner's principle may be used. Fincham (un- 

 published observations) has also made use of a 

 photoelectric cell with a feed-back which auto- 

 matically focuses the instrument. 



Campbell (15) has designed an optometer based 

 on the use of a photocell and Scheiner's principle 

 which automatically records changes in accom- 

 modation. 



The indirect ophthalmoscope may be used like a 

 coincidence optometer except that the blood vessels 

 and demarcations on the retina are used instead of 

 an image of an external target focused on the retina. 

 In this way only the focus of the emerging beam is 

 involved in the measurement. 



With the direct ophthalmoscope the refracting 

 mechanism of the subject's eye is used as a magnifier, 

 the focal length of which can be varied with the 

 auxiliary lenses mounted in the instrument. 



The aberrations of the eye and the differences in 

 criteria as to what constitutes a focus create a problem 

 in trying to correlate the results obtained with sub- 

 jective and objective methods. 



A technique known as the fogging method has been 

 developed for measuring the 'zero level' of ac- 

 commodation by manipulating the stimulus pattern 



Fig 19 



FIG. 18. Scheiner principle with doubling as the criterion of the retinal image being out of 

 focus. 



FIG. 19. Scheiner principle with vernier displacement as the criterion, .\ssume that the upper 

 and lower halves of the slit are covered with polaroid with the axes at right angles, and that the 

 two holes in the viewer are also covered with polaroid with the axes crossed at right angles. 



