334 THE SPECIAL SENSES. 



retina of the observer. The concave lens necessary to give this result, plus 1 D, 

 for distance, gives the extent of the myopia in diopters. With astigmatic 

 eyes the point of reversal may be determined for the different meridians 

 of the eye, the movements of the mirror being in the same meridian. By 

 the character of the reflected spot and the points of reversal it is possible 

 with the retinoscope to determine the principal meridians, and the difference 

 in refraction between them, that is, the degree and the axis of the astigmatism. 

 The Ophthalmometer. The ophthalmometer is an instrument for 

 measuring the curvature of the refracting surfaces of the eye. As actually 

 applied in practise it is arranged especially for measuring the curvatures 

 of the cornea along its different meridians. The point for which the instru- 

 ment is designed is to obtain the size of the image reflected from the convex 

 surface of the cornea. Any luminous object placed in front of the eye will give 

 a reflected image from the cornea as from the surface of a convex mirror. 

 If the size of the object and its distance from the cornea are known and the 

 size of the corneal image is determined, then the radius of curvature of the 



2 in, 

 cornea is given by the equation r = -~. t in which p represents the distance of 



T. 



Fig. 139. Schema to indicate the general principle of the ophthalmometer : T, 

 Telescope to observe the reflected images from the cornea ; A and B, the targets or mires 

 in the shield at a known distance apart whose images are reflected from the cornea ; a 

 and b, the reflected images of A and B on the cornea. The distance a-b has to be determined. 



the object from the cornea, i, the size of the corneal image, and o. the size 

 of the object. For example, let A and B in Fig. 139 be two luminous areas 

 arranged on the arc of a circle. If placed in front of the cornea C each 

 will give a reflected image a and b, which may be observed by means of 

 the telescope T. The distance between A and B represents the size of 

 the object and the distance between a and b the size of the image. This 

 latter factor is determined by means of the telescope. A scale, for in- 

 stance, might be placed in the eye-piece of the telescope and the distance 

 a-b be determined in terms of its graduation. This valuation might then be 

 converted into millimeters by substituting a scale for the cornea and measuring 

 off upon it the observed distance in the eye-piece scale. If the arc carrying 

 AB is arranged so that it may be rotated it is obvious that the size of the 

 corneal images may be measured for the different meridians and thus enable 

 one to compare their curvatures. In modern instruments, such as is repre- 

 sented in Fig. 140, the luminous areas, known as targets or mires, are placed in a 

 spherical shield which may be rotated around the axis of the telescope. The 

 shield has a radius of curvature of 0.35 meters and its center of rotation is 

 approximately coincident with that of the cornea when the eye is in its 

 proper position. The reflected images of the mires from the surface of the 



