ERRORS OF REFRACTION AND ACCOMMODATION. 151 



diagram of the course of the red rays and of the violet ones, 



using the schematic eye, and explain the apparent contradiction 



between the results of this and of the preceding experiment. 



When we turn our eyes to look at a certain object we habitually 

 place them so that the image falls on the fovea centralis. This is the 

 part of the retina which is capable of seeing most acutely, of making 

 out differences smaller than can be perceived by any other part, and 

 the line which connects it with the nodal point is called the VISUAL 

 AXIS. If this corresponded with the optical axis the refraction 

 would be the best that the refracting media of the eye are capable 

 of. In point of fact, however, there is an angle of about 5 between 

 the two and this is a slight additional source of error in refraction 

 in the eye. 



ERRORS TENDING TO BE PATHOLOGICAL. The defects in refraction which have 

 been described so far are found in every eye. As well as these there are several 

 types of faulty formation of images which are often found, any one of which if 

 it is at all pronounced makes the vision of the eye abnormal. 



NEAR-SIGHTEDNESS OR MYOPIA. In near-sighted eyes light from distant 

 objects, that is for practical purposes parallel light, is brought to a focus in front 

 of the retina instead of exactly on it. The rays therefore when they reach the 

 retina are already diverging from their focus and they form a blur on the light- 

 sensitive layer instead of a sharp point of light (Fig. 42 B.). This may be 

 because the curvature of one or all of the refracting surfaces is unusually sharp, the 

 length of the eyeball being normal, or it may, as is more frequently the case, be 

 due to an unusual length of the eyeball. There is no physiological correction for 

 this condition. In practice it is rectified by placing in front of the eye a ccncave 

 lens. Light from distant objects is thus made divergent before it reaches the eye 

 and its focus therefore lies on the retina, further back than the principal plane of 

 the shortsighted eye. 



In LONG-SIGHTEDNESS OR HYPERMETROPIA the condition is reversed. The 

 principal focus of the refracting media lies behind the retina and the parallel rays 

 from distant objects have not yet arrived at their focus when they are interrupted 

 by it (Fig. 42, C.). An increase in the refractive power of the eye 

 is needed to bring the focus forward and give a clear image. An effort of contin- 

 ued accommodation (see below) will accomplish this, but such an effort gives rise 

 to various nervous symptoms, headache, irritability and the like. If convergent 

 glasses of suitable strength are used, the focus may be advanced the necessary 

 amount without any effort on the part of the patient. 



ASTIGMATISM. So far as we have considered the eye as refracting all rays to 

 the same extent no matter in what plane they diverge from their object nor 

 whether they pass through the upper and lower parts, or through the right and 

 left sides, of the refracting surfaces. Cases are fairly common, however, in which 

 this does not hold good. In these eyes, known as astigmatic, all the meridians 



