2 



Dr. T. Reid. 



[Feb. 16» 



Let SP be an object situated at the principal focal distance of MM', 

 and let XX' be the principal axis of the system. 



Then a ray SM parallel to the axis will, after refraction, be 

 directed to the principal focus P' of the curved surface of the cornea, 

 and therefore be reflected in the direction IB parallel to the axis XX'. 

 IB prolonged will meet the ray directed to the centre C at the point 

 S' ; therefore S' is the image of S, and S'P' the image of SP, and 

 S'P' is in the principal focus of the convex reflecting surface. 



In the similar triangles MP'O and IP' A, 



P'O _ MO 

 P'A~ IA' 



SM and IS', the prolongation of the reflected ray, are parallel to the 

 axis XX', 



therefore SP = MO and S'P' = IA. 



Therefore 



P'O SP 

 P'A " S'P' 







or j 





/ = 



r 



2 5 



therefore 







I ~ 



2F 



r 



therefore 





2PxI 

 ~~ 6 



F 



Description of the Instrument. 



The essential parts of the instrument are an aplanatic convex lens 

 of known focus, a rectangular prism neutralised in its centre by a 

 smaller prism, one side of the rectangular prism being adjacent to the 

 lens and a circular or other disc being opposite the other side in the 

 principal focus of the lens. When the instrument is held in front of 

 the convex reflecting surface with the disc turned towards a luminous 



