THE TEST SET 7^7 



moscope. Seat the patient erect with his head supported in the head rest 

 and eyes level with the instrument and one covered. Set the pointer of 

 the telescope at 90 and focus for a clear image of the mires from the corneal 

 surface of the exposed eye raising or lowering the telescope if necessary. 



Turn the telescope until the meridian lines of the adjacent images form 

 an unbroken line and the spurs an exact cross. This is the primary posi- 

 tion and the radius of curvature is read off the side scale in millimeters 

 and tenths. 



Set the primary position indicator on the left side wheel at "o," then 

 turn the telescope through 90 degrees or until the secondary position is 

 found. If there is no astigmatism the cross will still be perfect. If there 

 is astigmatism then adjust the right hand wheel to reform the cross and 

 read the right scale as before and compute the astigmatism. 



29. Visual Acuity. The localizing power of the retina is measured by 

 the angle formed at the nodal point by the rays from the opposite borders or 

 limits of an object that can just be identified. The standard test is based on 

 the size of an object at a distance of twenty feet which will subtend an angle 

 of one minute. The letters of the standard test chart are constructed on a 

 total angle of five minutes, but the identifying marks subtend an angle 

 of one minute. The visual acuity of the eye should be tested first for 

 the right eye, then for the left. Hang a test chart at a distance of twenty 

 feet, so that its disc is well illuminated, and allow the individual tested 

 to read off the letters on the chart, beginning with the larger ones at the top. 

 The letters on this chart are constructed on the basis of a visual angle of 

 five degrees. When the letters marked "twenty feet" or "six meters" 

 represent the limit of accurate identification,, the visual acuity is said to 



be , i.e. i, or normal. If the line marked " thirty feet" is the limit that can be 

 20 



read at the normal distance of twenty feet the eye is subnormal and the visual 



acuity is measured by the fraction , i.e. 2/3 that of the standard. If 



3 



20 

 the "fifteen feet" test can be read, then the visual acuity is , or 4/3 the 



standard. 



If the eyes tested are astigmatic, or have other optical defects, these 

 must first be corrected before testing for visual acuity. 



30. The Test Set. The student is recommended to close the experi- 

 ments on the eye by fitting glasses for himself and at least two others. He 

 should correct for the defects that have been revealed in the preceding experi- 

 ments, especially for astigmatism; myopia, or hypermetropia; and presby- 

 opia. Of course each eye must be tested and fitted separately. 



