858 



THE SENSE OF SIGHT 



The presence of astigmatism may be revealed by looking at a chart 

 such as is represented in Fig. 455, because an astigmatic eye is unable 

 simultaneously to obtain a perfectly clear image of lines placed at 

 right angles to one another. An even more delicate test is presented 

 by the concentric rings reproduced in Fig. 456. It should be empha- 

 sized, however, that the oscillating blurring effect which one frequently 

 obtains while gazing at these charts, is not caused by an astigmatic 

 condition of the refracting media of the eye, but by slight variations in 

 the degree of contraction of the ciliary muscle. Such variations must 

 necessarily give rise to changes in the accommodation. 



FIG. 458. OPHTHALMOMETER. 



An instrument which enables us to determine the direction as well as the degree 

 of the excessive curvature of the cornea, is the ophthalmometer of Helmholtz (Fig. 

 458). It is constructed in such a way that the size and shape of the corneal image 

 of any luminous object may be determined with absolute accuracy. Knowing the 

 size of this object and its distance from the eye, as well as the size of the corneal 

 reflection, it is possible to ascertain the radius of curvature of the cornea according 



2z*t 

 to the equation r=^-.. In this formula p represents the distance of the object 



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

 need scarcely be mentioned that the reflecting surface and telescope of this instru- 

 ment may be rotated so as to enable the observer to measure the curvature of the 

 other planes of the cornea and to compare them with one another. In the modern 



