ADVANCED EXPEEIMENTAL PHYSIOLOGY 



367 



A modification of Helmholtz's ophthalmometer was introduced by 

 Javal & Schiotz, in which the double glass plate was replaced by a calc- 

 spar crystal and a similar double image obtained. This was still 

 further improved by Kagenaar, who substituted compound prisms for 

 the crystal, and the instrument so cheapened and improved is generally 

 spoken of as an astigmometer. This instrument, which is essentially an 

 ophthalmometer, is represented in Fig. 241. 



Fig. 241. The ophthalmometer. 



It consists of a telescope, which is directed towards the subject's eye, 

 the head of the subject rests in the frame, opposite the telescope. The 

 eyepiece of the telescope is first adjusted by focusing a thread which 

 lies in the plane of the image formed by reflection from the cornea. 

 This adjustment is carried out by turning the telescope towards a 

 milk glass plate on the left of the subject, and moving the eyepiece till 

 the thread is defined. The telescope is then directed towards the 

 subject's eye, and moved with its stand backwards or forwards towards 

 the observed eye till either of the reflected images of the illuminated 

 areas on. the quadrant is clearly defined. In the quadrant is a fixed 

 area opposite a white line corresponding to the number 20 on the scale. 

 Let the quadrant be first placed in a horizontal plane, with the fixed 

 illuminated area to the left. According to the varying position of the 

 right illuminated area two pairs of images will now be seen reflected 

 from the cornea, and attention should be directed to the two middle of 

 these images, which may or may not overlap (Fig> 242). The right 

 moveable area should now be adjusted on the quadrant so that the edge 

 of one image just touches the edge of the other, the * stepped ' image 



