104 



EXPERIMENTAL PHYSIOLOGY 



case, when the needle is nearer to or further from the near or far 

 points of distinct vision, its image appears not blurred but double. 



That the eye cannot obtain a sharp image simultaneously of a 

 near and a distant object is shown by taking two needles and fixing 

 one at about five inches along the scale and the other some inches 

 further. If now, in Schemer's experiment, the eye is focused on the 

 near needle the far one looks double, and vice versa. 



FIG. 82. BOARD WITH PERFORATED CARD FOR SCHEINER'S EXPERIMENT, a, 6, NEEDLES ; 



c, PERFORATIONS IN CARD. 



Changes in the pupil produced by drugs. 1 Carefully enucleate 

 the eyeballs of a frog which has just been killed and place each in 

 a watch glass of Ringer's solution. Measure the diameter of the 

 pupils with compasses and note down the size of each. Add to the 

 contents of one watch glass a drop or two of extract of suprarenal 

 capsule or of dilute solution of eserine or of atropine. After a short 

 time again measure the pupils. Repeat the measurement after an 

 hour or more. 



The effects of drugs are also investigated, in man or animals, by 

 dropping them on the conjunctiva, and, in animals, by injecting 

 them into a vein. 



The Ophthalmoscope. This instrument, which is used for examining the 

 interior of the eyeball, consists essentially of a small concave mirror with a 

 hole in the centre. (For the indirect method a biconvex lens is also required.) 

 Practise first on an artificial model of the eye and then on the living subject. 

 It can be practised upon a rabbit : a drop or two of a 1 per cent, solution of 

 atropine should previously be instilled into the eye. Or a frog, with the body 

 wrapped in a cloth, held up near the source of light may be employed. 



Direct method. Only a limited part of the retina is seen at one time, but it 

 is much magnified. The subject is seated in a darkened room with a light, not 

 too bright, near his ear. The observer sits in front of, and on a slightly higher 

 level than, the subject, close to him. The observer holds the mirror in front 

 of and close to his own eye, and, throwing the beam of light into the subject's 



1 The effects upon the pupil of cutting and stimulating the sympathetic have 

 already been studied (see p. 82). 



