428 Mr. J. Bridge on the Measurement of 



Make the zero point. The lengths of the divisions are to 

 be such that CO shall contain 1000-f-/ of them. 



[In the actual apparatus / was 5J in. and (70 contained 

 190 divisions, shown in the figure as 19.] 



L is a concave lens of focal length/' which may be inserted 

 at F. This has the effect of increasing the length of the scale 

 by moving its zero to a greater distance. 



G is an inclined plane on which is placed the object to be 

 viewed. The object which I usually prefer is a series of 

 equal black circular spots with narrow white intervals. The 

 middle interval has a mark by which it may be recognized, 

 and it is helpful that every 10 divisions of the scale should 

 correspond to exactly one spot or exactly two. Being looked 

 at very obliquely, the spots appear as very flat ellipses with 

 minor axes about a fifth of their major axes, separated at the 

 extremities of their minor axes by very narrow white spaces. 



3. Let P be a point in the object at distance u from the 

 lens, Q the image of P, and let FQ = r. Then 



1 11,1 f-u 



1. = 7 ; whence - = J — H - . 



v-f u f v f 



Consequently the divergence of the refracted pencil as it 

 passes F and enters the eye is indicated by the number on the 



scale under the luminous point. For, an inch being ' 2 



.... ,, . . , . 1000 , , , 1000 ,. . . / 

 divisions, (j—u) inches is —^- (f—u) or divisions. 



Each division represents an amount of divergence equal to 

 that of a pencil whose vertex is 1000 inches distant. 



Applying this to the object on the inclined plane G when 

 its position is that shown in the figure, we see that its points 

 are viewed by means of converging pencils varying in con- 

 vergence from to 180 units of the scale. 



4. The apparent magnitude of the object is unaltered by 

 the motion of the table, the eye being supposed at F. For 

 the rays which go through that point from two given 

 points of the object are before incidence parallel to the axis, 

 and strike the lens at points which are not changed by 

 the motion of the table. Hence we see that the arrangement 

 enables us to offer to the eye an object which gives on the 

 retina an image of constant size and shape, but which makes, 

 in its different parts and in its movement, a continuously 

 changing demand on the power of accommodation. 



5. The range of convergence which can be offered to the 

 eye is from O to B, and that of divergence from O to C. It 



