1062 



THE SENSES 



horizontal limbs of a cross cut out of a piece of cardboard and placed 

 in the path of the beam of light cannot be both focussed at the same 



"^'Astigmatism (Regular). (i) Look at a figure showing a number 

 of lines radiating horizontally, vertically, and in intermediate directions 

 from a common centre. First fix the figure at such a distance that one 

 can comfortably accommodate. If astigmatism is present, all the lines 

 cannot be Seen with equal distinctness at the same time, but they can 



all be successively accommodated for. 

 Next, bring the figure to the near- 

 point of distinct vision for the hori- 

 zontal and neighbouring lines. Prob- 

 ably the vertical lines will be blurred 

 and cannot be made as distinct as the 

 horizontal by any effort of accom- 

 modation. If the eye is distinctly 

 astigmatic, the difference will be 

 marked. 



(2) Use the Ophthalmometer. A 

 convenient form is shown in Figs. 453 

 and 454. 



Raise or lower the chin-rest till the 

 upper bar of the head-rest is just 

 above the patient's eyebrows, his head 

 being exactly vertical. The eye not 

 to be examined is covered with the 

 blind . The patient looks steadily into 

 the opening of the tube with his eye 

 wide open. The height of the instru- 



Fig . 45 ^_0phthalmometer, as seen P ent Caving been adjusted, a clear 



image of the mires is obtained by 

 focussing. The tube is then turned 

 horizontally slightly to right or left 

 until the two images of the mires are 

 close together and equally distinct. 

 Rotate the outer tube (Fig. 454, d) 

 until the long meridian lines of the 



C, milled head for raising and lower- i maRes are exactly in line with each 

 ing chin-rest; F, milled head for * ,, . _j_- - 



from behind the Patient. B, blind 

 for covering the eye not being ex- 

 amined; H, chin-rest; A, A, gradu- 

 ated discs on which radii of curva- 

 ture of the cornea in various meri- 

 dians are read off or their equivalent 

 in diopters ; E, eye -piece of telescope ; 



adjusting height of the ophthalmo- 

 meter, and G for moving it horizon- 

 tally back and forth; n, graduated 

 disc for giving the rotation of the 



other. If there is no astigmatism, 

 this will be seen at all axial posi- 

 tions ; if there is astigmatism, at only 

 two positions. An axis having thus 



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outer tube of the telescope and the been obtained, the graduated disc 

 black disc u. In u are seen the two (Fig. 453, A) on either side of the 

 illuminated mires. tube is rotated until the shorter lines 



or spurs of the images also unite, 



forming a perfect cross with the longer ones (Fig. 455), and the adjust- 

 able pointer on the left-hand disc is made to coincide with the stationary 

 one and a reading taken. Now rotate d through 90 degrees; the long 

 axial lines of the images will be in alignment without further adjustment. 

 But if the eye is astigmatic, the short lines will not (Fig. 456). By 

 rotating A, the short lines are made to coincide, so that a perfect cross 

 is again formed, and the graduation is read. The difference between 

 this and the previous reading i.e., the difference between the two 

 pointers gives the difference in the curvature of the cornea in the two 

 meridians. The images of circles which form the outer portion of the 

 mires are oval in ordinary astigmatism. 



