NEAR AND FAR LIMITS OF VISION 759 



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

 perpendicular. 



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

 ter'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 of 45 degrees and in line 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 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. Draw two sharp 

 lines at right angles across a white sheet of paper. Lay a block of glass 

 with polished surfaces on the paper with one margin coinciding with the 

 horizontal line. Insert one pin at the intersection of the lines but in con- 

 tact with the glass. Insert a second pin at the opposite margin of the 

 glass and at an angle of about 40 degrees from the intersection of the lines 

 on the paper. Now looking through the glass determine a third point 

 out on the paper away from the glass and where the two pins seem to be 

 in line. Remove the glass and construct a diagram showing the angles 

 of incidence and of refraction. 



The index of refraction is found by the formula: 



sin i 



= Refractive index 



sin r 



If equal distances are laid off on the hypotenuses then one may substitute 

 the actual measurements of the vertical legs for the sins in the formula 

 and calculate. Repeat several times. 



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 fur- 

 nished from the optical set. P.ut 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 refrac- 

 tive power of lenses numbers 2, 3, and 4. If a source of light of a finite 

 distance is used, then correction must be made for the divergence of the 

 rays by the formula: 



1 + - - - 1 - 

 f i' " F 



Where f and f are the conjugate foci and F the principal focal distances in 

 meters. 



