ASTIGMOMETER

n.?g-mo%m/e%t-u'r. From a priv., oriypa, a spot, and pérpoy, à measure. Fr., astigmomètre. An instrument for measuring the degree of astigmatism by determining the difference in refraction between the meridians of the cornea, devised by Javal and Schiotz, and based upon Coccius’s ophthalmometer. The object of fixation is formed by two white rectangles moving on a perimetric arc, with a radius of 35 ctm.,. in the centre of which is the corneal image of the eye observed. This reflection is seen through a lens or gla , the objective of which pred rcee real and reversed image in the tube of the instrument. ‘his image is divided by a Wollaston's bi-refracting prism, introduced between the two objectives and seen through the eye-piece, at the focus of which it is formed. The strength of the bi-refracting prism is such that it exactly bisects an object of 3 mm., situated at the point where the eye of the patient is to be placed. One of the white rectangles is cut out in the shape of a staircase in which the steps measure 6 mm. in width. The instrument being properly ppc the two rectangles may readily be turned until one of the mages of one is in contact with one of the images of the other. In turning the instrument around its axis, the two images maintain their relative position if no astigmatism is present. hey separate or become superimposed if the curvature of the cornea varies from one meridian to another, Each step of the staircase, one of the images of which encroaches upon the other, when the position of the arc changes by 90°, corresponds to a refractive difference of D1 between the two meridians of the cornea, and a fraction of a step to an equivalent fraction of a dioptre. [F.] Cf. Registering KERA- TO SCOPE.