522 



REPOKT — 1881. 



the object, so that in looking at a distant object they are directed parallel, and in 

 looking at a near one they converge. These movements are effected by the 

 external muscles of the eyeball, v^hich are supplied by branches of the same nerve 

 as the ciliary muscle. As a fact these movements of the ciliary muscle and of the 

 external muscles of the eyeball are associated, or habitually performed in conjunc- 

 tion ; that is, the brain has become accustomed to send an impulse to the one set of 

 muscles proportionate to that sent to the other. Any disturbance of this associa- 

 tion can only be accomplished by a distinct effort which, if severe or long continued, 

 is apt to be painful. 



Suppose a man has become presbyopic, i.e. his accommodation has gradually 

 become stiff, and its range reduced. In order to accommodate for rays from an 

 object at the ordinary reading distance of ten or twelve inches, he has now to 

 exert an effort equal perhaps to what he would have employed when young on 

 one four inches off, but the change has been gradual, and the convergence of 

 the eyes for twelve inches has become associated with this amount of effort. If 

 he now use convex glasses of suitable power, the want of refracting power is 

 supplied, the effort of accommodation is reduced to 

 its natural amount, but the amount of convergence 

 which has become associated with this small effort is 

 now insufficient, and the eyes, instead of converging 

 to twelve inches, converge on a point several feet dis- 

 tant, so that double vision would be produced, unless 

 by a distinct effort the eyes were converged more, and 

 this effort is often painful and is expressed by the 

 term that the spectacles ' draw ' the eyes. After a 

 time new associations are formed and the spectacles 

 can be used comfortably, but this does not happen in 

 all cases, and for these it is necessary to grind the 

 lenses on glasses of prismatic section. The action of 

 the prism is so to bend the pencils of rays coming 

 to the eyes that they appear to diverge from a point 

 corresponding to the new focal distance of the eyes 

 provided with the spectacles. 



Sometimes the amount of prismatic effect required 

 is calculated, but the calculation being baaed on gene- 

 ral considerations does not always suit individual 

 persons ; at other times prismatic glasses from a trial 

 case are combined with the calculated spherical, or 

 spherical and cylindrical glasses, until one is found 

 with which vision is comfortable. 



In many cases it is not necessary to use glasses spe- 

 cially ground on prisms, but suihcient to move the 

 centres of the glasses nearer together. The glass being 



thicker in the centre, looking through the part near the edge produces an amount 

 of prismatic effect which is often sufficient. If concave glasses are used, as in 

 cases of short sight, then they must be further apart than the distance of the eyes 

 in order to produce this effect. 



The object of the instrument exhibited is to find experimentally the amount of 

 prismatic power, and the distance of the centres of the lenses, which is required in 

 any individual case. 



Two circular frames each 2^ inches in diameter, and with teeth cut in their 

 edges, are mounted so that the teeth gear into each other, and they can rotate 

 freely, but in opposite directions. In the centre of each frame is mounted a prism 

 of 18° ; one of the frames is graduated, and when the graduation is at 0° the axes of 

 the prisms are parallel, so that parallel pencils of rays falling on both are deviated 

 both in the same direction, and still parallel. Thus when the pair of prisms are 

 arranged horizontally in front of a pair of eyes, an object looked at appears dis- 

 placed up or down, but there is no lateral deviation on either. If the frames be 

 rotated 90° in one direction, the prisms both have their bases inwards, or, if in the 



