4 



Dr. T. Reid. 



[Feb. 16, 



fixation point and his visual line is vertical to the point of the cornea 

 through which it passes, the corneal image doubled and inverted 

 ought to be seen in the centre of the field. Instead of using circular 

 discs of different dimensions, the size of image required to produce 

 exact contact in any meridian is conveniently and quickly obtained 

 by making the required change in the size of a carefully coustructed 

 iris diaphragm. By using a circular object, the circular, elliptical, 

 or irregular form of the image reveals at once the condition of the 

 surface. When the image is elliptical, the meridian of greatest 

 curvature is easily found by rotation of the telescope, and a rotation 

 of 180° gives a controlling observation. By a similar process the 

 meridian of least curvature is determined. 



Graduation of the Instrument. 



Let D be the power in dioptres of the cornea as a refracting 

 surface, with a medium behind it of uniform density having an 

 index of refraction n = 1*337 approximately. 



(n — 1)1000 



D = 



r 



337 

 r 



(ii). 



Combining equation (I) with (II), 



_ 337 X O 

 "'"SET* 



_ , 337x0' 



D+1 = ^iF-' 

 x = |f F (0'-0). 



In the present instrument 1 = 2, and 2F = 52, 

 therefore 1 = 3*24 (O'-O), 



- 1 - = O'-O; 

 3*24 



therefore ID = rather less than J mm. 



The index is divided into two parts, outer and inner. The outer 

 registers the size of the image, and the inner the corresponding 

 dioptres. 



The degree of refinement with which the measurements may be 

 carried out depends entirely on the degree of exactness of determina- 



