LXXVI.] TOUCH, SMELL, TASTE, HEARING. 367 



upon the patient's eye, telling him to look, not cU the mirror^ but at the paper 

 placed upon the wire. The paper must be on the opposite side to the eye. 



(5.) Take the rod in the hand and adjust the position of the lens so as to 

 bring the optic disc into view. 



(6. ) In changing places with another observer, cut oflf the light from the 

 mirror by means of the can die -screen. 



7. Intraocular Pressure, — Pick's Ophthalmotonometer. 



This instrument is extensively used in German eye-hospitals, and consists ot 

 a small brass plate (6 mm. diameter), which is attached by means of a metallic 

 spring to a base, which also carries a scale which indicates the amount of 

 pressure applied. One presses the disc of the instrument against the eyeball 

 until it flattens the part to which it is applied, when the pressure is read off 

 in grammes. The experiment may be done first on a rabbit, as most of them 

 remain quite passive. Place a person with his left shoulder next the window, 

 ask him to turn his eyeballs to the right and open his eyelids, whereby 

 suflBcient of the eyeball is made visible for the application of the instrument. 



8. The Pupil. —Normally the pupil in man, rabbits, and other animals is 

 black, but in albinos it is reddish. Why ? 



(i.) Select an albino rabbit, and exactly in front of its pupil hold up a black 

 card with a hole in it the size of the pupil. Direct the pupil to the light, and 

 arrange the shade so that all light is kept from the eye except that which 

 enters it by the pupil. The albino pupil then appears black. 



This shows that the blackness of the pupils is not due to the light 

 entering the eyeball being absorbed by the pigment of the fundus of the eye, 

 but that light entering the eye can only emerge by the pupil when the iris and 

 the neighbouring parts of the choroid, in virtue of their pigmentation, do not 

 permit light to pass through them. The construction of the dioptric apparatus 

 of the eye is such that light from the fundus of the eye must be reflected back 

 to the source from which it came, i.e., to the focus. As we emit no light from 

 our Qje none can come to us from the observed eye, so we see the pupil black 

 oecause we do not illuminate the fundus with our body {Schenk), 



LESSON LXXVL 

 TOUCH —SMELL— TASTE— HEARING. 



1. Touch — The Sense of Locality. 



(a.) Ask a person to shut his eyes, touch some part of his hody 

 with a pin, and ask him to indicate the part touched. 



(//.) ^sthesiometer.— -Use a small pair of wooden compasses, 

 or an ordinary pair of dividers with their points guarded by a 

 small piece of cork, or Sieveking's ^sthesiometer. Apply hghtly 

 the points of the compasses simultaneously to different parts of 

 the body, and ascertain at what distance apart the points are felt 

 as two. The following is the order of sensibihty : — Tip of tongue 



