THE DIOPTRIC SYSTEM. 



1027 



In Fig. 371 an observer at I will see through the plate aa as if it 

 were situated at 0', and through the plate lib, at 0". An object seen 

 through the two plates will appear doubled, the double images 

 deviating in opposite directions. The amount of deviation of each 

 depends on the thickness of the plate li 

 on the angle of incidence, and on the 

 refractive index. 



cos p 



If the angle the plates make with one 

 another be so adjusted that the two images 

 just touch at their inner edges, the dis- 

 tance O'O" will be equal to the diameter 

 of the object, each image having been 

 moved outwards through half its diameter. 



Radius of curvature of the cornea. 

 In order to find the curvature of the 

 cornea, the eye to be examined is placed 

 about 2 metres from the ophthal- 

 mometer, in front of which is a screen 

 bearing two luminous objects, a and I, in 

 Fig. 372, the distance between a and I 

 being about half a metre. Four points 

 of light will be seen reflected from the 

 cornea, and the plates are adjusted till 

 the two middle points coincide, so that 

 only three points are seen. The size of 

 the image |3 may be calculated by the 

 formula given (or more commonly in 

 practice is found from a table prepared 

 for the instrument), and the radius of 

 curvature of the cornea is calculated from 



2x8 

 the formula r = where x = the dis- 



y 



tance between the cornea and the object db, and 7/ = the size of 

 the object, namely, the distance between a and I. By this method 

 Helmholtz found the radius of the central part to be 7'829 mm. Other 

 investigators have given rather smaller figures. 



c ' I " 

 FIG. 371. 



FIG. 372. 



In another form of ophthalmometer, devised by Coccius, the plates are 

 replaced by a double image prism, and the images are brought into apposition 



