252 VISION. 



They are divided into degrees, and each carries a curser 

 C bearing a piece of white or coloured paper. 



The operation is to be conducted with movable lights in a 

 darkened room. 



The subject throughout the observation looks directly with 

 one eye at the point of fixation F, which is a mirror with 

 cross lines. He knows that his eye is centred when the 

 cross is centred in the reflection of his pupil. 



His chin must rest on R (suitably adjusted for height). 



The cursors are moved in succession from the equator 

 towards F by the observer until just visible to the subject. 



Determine successively the outlines of the fields of the two 

 eyes for white, red, green, and blue and transfer the results 

 to a printed chart. 



Remember that the point of fixation corresponds to the 

 fovea centralis of the retina. The retinal image of the field 

 is reversed in all directions. 



Compare the fields of the two eyes. 



27 Examination of the interior of the eye. The ophthal- 

 moscope. 



Light entering the eye is reflected from the retina in the same 

 direction as that from which it entered. You cannot, therefore, 

 see into another person's eye without employing some artifice, 

 because your head intercepts the light. 



Artificial eye : A round (pill) box of about 22 mm depth, 

 blackened internally, in the lid of which is a lens with 

 a principal focus of the same length as the depth of the box. 

 The bottom of the box represents the retina ; it has some 

 printed matter upon it, and three small apertures through it, 

 placed l-5 mm from each other. The latter can be blocked 

 at will with a piece of black card, so that the apertures 

 appear as black dots when viewed from the front. 



