. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 131. frame. At the side of the frame, and attached to it, arc twoor four electric lights or Argand burners, which illuminatethe mires. The surgeon, looking through the eye-piece ofthe telescope, focuses the center of the patients corneauntil he sees two images of each mire cKarl\- ; tlun the two central imaiies for further studv and isjnores ASTIGMATISM. 149 the peripheral imaL^cs. The next step is to move theright-hand mire until these two images of th
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 131. frame. At the side of the frame, and attached to it, arc twoor four electric lights or Argand burners, which illuminatethe mires. The surgeon, looking through the eye-piece ofthe telescope, focuses the center of the patients corneauntil he sees two images of each mire cKarl\- ; tlun the two central imaiies for further studv and isjnores ASTIGMATISM. 149 the peripheral imaL^cs. The next step is to move theright-hand mire until these two images of the mires occupythe center or pole of the cornea, so that their inner edgesjust touch and the black line in each makes one continu-ous black line through both (see Fig. 133) ; and to do the. Fig. 132. latter, the barrel of the telescope may have to be graduallyrevolved from left to right or right to left, but never morethan 45 degrees either way. When this position is ob-tained, the axis or meridian is noted by the arrow, whichpoints to the figure on the dial at the back of the arc, or,as in some old instruments, on the front of the dial. This ISO REFRACTION AND HOW TO REFRACT. position of the mires is spoken of as the primary posi-tion. Revolving the telescope to the opposite meridian (mer-idian at right angles), which is called the secondary posi-tion, the observer notes any change which may have takenplace in the relative positions of the mires. If they havenot changed, but still maintain their edges in apposition, asin the primary position, then the cornea has a uniform cur-vature thoughout, and there is no astigmatism of thecornea present. If, however, when the secondary positionis reached and the catoptric image of the mires with thesteps has encroached upon the cat
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