120 



EXPERIMENTAL PHYSIOLOGY 



and more striking by Schemer's device of observing the needle 

 through two pinholes made close to one another and side by side in 

 a card fixed vertically at one end of the scale (Fig. 89). In this 

 case, when tjie 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 simultaneously obtain sharp images of a 

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

 one at about 5 inches along the scale and the other some inches 

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

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



Changes in the pupil produced by drugs. 1 Carefully enucleate 

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



FIG. 89. Board with perforated card for Scheiner's experiment, a, 6, Needles ; 

 c, perforations in card. 



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 of extract of suprarenal capsule. 

 After a short time again measure the pupils. Repeat the measure- 

 ment after an hour or more. Dilute solutions of eserine and of 

 atropine can be similarly tested. 



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. Direct method. The instrument which is 

 used for examining the interior of the eyeball consists essentially 

 of a small concave mirror with a hole in the centre. 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. 



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

 already been studied (see p. 90). 



