ELEMENTARY EXPERIMENTAL PHYSIOLOGY 235 



Points of Accommodation. Line of Accommodation. At a certain 

 distance close to the e} r e the power of accommodation is lost. 



EXPERIMENT I. Hold a needle about 2 feet from the eye and gradu- 

 ally bring it nearer ; it is for a certain time possible to obtain a clear 

 image. At a certain distance, in spite of effort, the image begins to 

 get blurred. The least distance at which one obtains a clear vision of 

 the needle corresponds to the near point of accommodation. This is 

 generally about 8 inches. In short-sighted persons a far point of 

 accommodation may also be shown. If the distance between the two 

 objects be not too great, although they are both in the line of sight, 

 they may be seen clearly at one and the same time. That is to say 

 that accommodation of a certain degree will enable the observer to see 

 objects at varying distances from the eye. The maximum distance at 

 which two objects in the line of sight may be separated will vary with 

 the distance of the nearer of them to the eye. As the nearer object 

 recedes from the eye the line of accommodation or the distance between 

 the two objects increases. 



[ADVANCED EXPERIMENT. Place two pins in the line of sight and 

 note the distance apart at which they are both visible as single objects 

 at the same time. Make observations with the nearer at 20 cm., 

 50 cm., 2 m. It will be found that the line of accommodation 

 lengthens with a greater distance from the eye.] 



3. Formation of Image in Excised Eye. The excised eye is accom- 

 modated for objects at a distance. 



EXPERIMENT. Remove the sclerotic and choroid from a fresh sheep 

 eye, and place it, cornea outwards, at the end of a cylinder of brown 

 paper. Direct it towards the window, and on looking down the tube 

 an inverted image of the window will be seen. 



This experiment can be still more easily performed on the eye of a 

 freshly-killed albino rabbit, which, for convenience of handling, should 

 be fixed in a ring of modelling wax or clay. In this case the sclerotic 

 and choroid are sufficiently thin to obviate the necessity for their 

 removal. 



4. Action of Iris in Accommodation, and its Changes with Variations 

 in Amount of Light. The iris cuts off the more peripheral rays imping- 

 ing on the cornea, otherwise the clearness of the image on the retina 

 would be diminished. This is especially the case when viewing near 

 objects, as here the angle of incidence of the circumferential rays is 

 greater. 



EXPERIMENT I. In not too bright a light direct the subject's attention 

 from a far to a near object. It would be noticed that the pupil becomes 

 smaller. 



