990 



A MANUAL OF PHYSIOLOGY 



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 instrument having been adjusted, a clear image 

 of the mires is obtained by focussing. The tube is then turned hori- 

 zontally slightly to right or left until the two images of the mires 

 are close together and equally distinct. Rotate the outer tube 



FIG. 437. VERTICAL SECTION OF OPHTHALMOMETER. 



d, outer tube of the telescope rotating in sleeve or collar s (supported by 

 standard t, which is swivelled in tubular support, g) ; k, diaphragm ; 10, eye-piece 

 with lenses a and b ; n, a stationary disc, borne on collar s, graduated to indicate 

 angle of rotation of u, a black concave disc rotating with tube d, and having 

 fixed in it two illuminated figures (or mires), w, w, whose images reflected from 

 the cornea are observed ; i is a pointer carried on the tube d which shows on the 

 graduated arc the amount of rotation ; 12, 12, hemispherical shells containing 

 small incandescent lamps for illuminating the translucent mires. The lamps are 

 connected with wires running in the hollow stem t ; f is the inner tube of the 

 telescope carrying the double prism, h, h. By means of the rack o, projecting 

 through the slot m, and engaged by the pinion p, f is moved back and forth 

 in the outer tube, thus approximating or separating the corneal images of the 

 mires. On the axis of p is a milled head for turning it, and two duplicate discs 

 graduated with a scale showing the radii of curvature of the cornea in millimetres, 

 and another scale showing their equivalent in diopters. 



(Fig. 437, d} until the long meridian lines of the images are exactly 

 in line with each other. If there is no astigmatism, this will be 

 seen at all axial positions ; if there is astigmatism, at only two 

 positions. An axis having thus been obtained, the graduated 

 disc (Fig. 436, A) on either side of the tube is rotated until the 

 shorter lines or spurs of the images also unite, forming a perfect 

 cross with the longer ones (Fig. 438), and the adjustable pointer 

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



