706 THE SENSES 



will suspend a very small drop. Too large a drop diffuses over too great an 

 area of the tongue. Occasionally small crystals of sugar, salt, etc., give 

 more satisfactory results. 



Perform the experiments on yourself before a mirror and map the re- 

 sults as shown in figure 481. 



If the experiments are done with care certain papillae will be found which 

 give one or two of the taste sensations, but not all. 



5. Sensations of Smell. Quantitative experiments on the sense 

 of smell are difficult to determine. Inhale vapor of ammonia so dilute that 

 it can just be detected. Note that the sensation is strongest at the moment 

 of drawing the vapor into the nostril. Fill the nostrils with the diluted vapor 

 and close the external opening; the sensation quickly disappears. Keeping 

 the nostrils closed, walk into the open air, then inhale fresh air. At the 

 moment of the inhalation of fresh air the ammonia is again perceptible. 

 Repeat with bergamot, rose water, etc. 



6. The Limits of the Sense of Hearing. Use a set of tuning forks 

 for the purpose, and determine the lowest vibration per second which can be 

 perceived as sound. Determine the highest limits in the same way. 



7. Acuteness of the Sense of Hearing. Listen to the vibrations of 

 a tuning fork, or, better, to the ticking of a watch which is moved back and 

 forth from the ear. Measure the distance at which it can just be distin- 

 guished. This experiment should be performed with the person blindfolded, 

 and extraneous noise should, of course, be suppressed. 



Refraction. Light passes out from a luminous point in straight 

 lines so long as the line of propagation is in a medium of uniform density. 

 If the rays pass from a transparent medium of one density into a second 

 medium of different density, they will usually be turned out of their course, or 

 refracted. If the rays enter the second medium at right angles to its surface, 

 they will continue in straight lines, but if they enter at any other angle they 

 will be refracted. If the second medium is denser than the first, the rays will 

 be refracted toward the perpendicular; if it is less dense, away from the 

 perpendicular. 



Use a Hall's refraction- measuring apparatus (constructed of a carpenter's 

 try square). Adjust it in a water-pan, and fill to the exact level with clear 

 water. Clamp a rule to the vertical limb of the apparatus at an angle with 

 the axial point of the instrument. Read the horizontal scale of the instru- 

 ment along the edge of the clamped rule. Remove the instrument from the 

 pan, using care not to disturb the adjustment of the ruler, and construct the 

 angle of refraction on co-ordinate paper. Determine the relation of the angle 

 of incidence and of refraction, and compute the refractive index of the water, 

 the air having a refractive index of one. 



Repeat the determination using a block of glass. Construct a diagram. 



9. To Determine the Refractive Power of a Convex Lens. Use a 



