218 EXPERIMENTAL PHYSIOLOGY 



CHAPTER XXII 



SOME EXPERIMENTS IN THE PHYSIOLOGY OF THE EYE. 

 ACCOMMODATION, OPHTHALMOSCOPY, COLOUE SENSE, PERIMETRY 



Accommodation. — An object can be seen distinctly if it be placed 

 close in front of the eye, or if it be removed to some distance from 

 the eye. But as an object can only be seen distinctly if its image be 

 accurately focussed on the retina, it follows that the eye possesses 

 some mechanism by means of which any image can at will be focussed 

 upon the retina. This means consists in a power of alteration of 

 the curvature of the lens. That this is the optical change pro- 

 duced may be proved by studying the images reflected from the curved 

 refracting surfaces of the eye, while the eye is fixed first upon a 

 near and secondly on a distant object. The power of throwing a 

 distinct image, now of a near object and now of a distant one, upon 

 the retina is termed accommodation. 



Before examining these images reflected from the eye make out the fol- 

 lowing points upon a series of watch glasses of different curvatures : — 



1. When light falls upon the surface of a medium of different refrangi- 

 bility from that in which it is travelling some of the light is reflected, even 

 though the medium be transparent. 



2. Hold a lighted match in front of a sheet of polished glass. An erect 

 image of the flame is observed of the same size as the match. 



3. Repeat, but use the convex surfaces of watch glasses of different degrees 

 of curvature, starting with nearly flat ones and choosing a series in which the 

 amount of curvature gradually increases. In all these cases images are pro- 

 duced which lie behind the glass, i.e. they are virtual, and which decrease in 

 size as the curvature increases. Note further that the images are erect, and 

 that, as they become smaller with the increased curvature of the surface, the 

 virtual image comes to lie nearer to the reflecting surface. 



4. Next examine the images produced from the concave surfaces of the 

 series of watch glasses. These are all inverted and lie in front of the sur- 

 face, i.e. they are real. They are all smaller than the object, and diminish 

 in size and move towards the surface as the latter becomes more curved. 



Now examine the images of a candle formed from reflection from 

 the eye. Hold the candle a little to one side and in front of the ob- 

 served eye and then look obliquely at the eye. One image is very 

 clearly seen : it is erect, small and virtual, and therefore comes from 

 a convex surface. The positions at which reflected images can be 



