THE LIMITS OF THE SENSE OF HEARING 671 



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 per-eptible. 

 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 distinguished. 

 This experiment should be performed with the person blindfolded, and ex- 

 traneous noise should, of course, be suppressed. 



8. 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 form 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 per- 

 pendicular. 



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 instrument 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 refrac- 

 tion on coordinate 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. 



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

 meter stick which is provided with a movable diaphragm or screen, and a 

 holder for a lens. Measure the focal distance of lens number i as furnished 

 from the optical set. Put the lens in its holder and focus the image of the 

 sun or. of an electric bulb on the screen, moving the screen back and forth 

 until the sharp focus is determined. If the lens is accurately ground, the 

 focus will be at a distance of one meter, which is the refractive power of a 

 one-diopter lens, by definition. In the same way determine the refractive 

 power of Jenses numbers 2, 3, and 4. 



