LXXI.] ACCOMMODATION. 335 



252, were placed one above the other, and could be rotated in opposite directions 

 round a vertical axis. One looks through the glass plates at two black lines 

 painted on a sheet of glass. On looking at the two lines through the two glass 

 plates, and on rotating the latter in opposite directions, one image is displaced 

 to the right and the other to the left, and the object appears double. One rotates 

 the plates until the inner edge of the one image coincides with the correspond- 

 ing edge of the other, so that each image has been disjjlaced exactly to the extent 

 of the size of the object. The size of the image can be calculated, provided one 

 knows the refractive index of the glass plates, their thickness, and the angle 

 formed by them. In the ophthalmometer the extent of rotation is read olf on 

 a disc placed outside the box which contains the glass plates. 



11. Line of Accommodation, z.e., the eye does not accommodate 

 for a point, but for a series of points, all of wliich are equally 

 sharply perceived with a certain accommodation. 



(a.) Stretch a white thread about a metre long on a blackened wooden 

 board. Through two narrow slits, about 2 mm. apart, in a blackened card, 

 focus with one eye a particular part of the thread, which must be in the optic 

 axis. A part of the thread on the far and near side of the point tocussed is 

 quite distinct and linear, but beyond or nearer than this the thread is double, 

 and diverges from the point focussed. 



{b. ) Make a small black spot with ink on a glass plate, and hold it in front 

 of any printed matter. Bring the eye as close as possible to the glass plate 

 without losing distinct definition of the point. At one and the same time 

 only one of the objects can be seen ; but not the point and the print equally 

 sharply defined. Remove the eye gradually from the glass plate, and ulti- 

 mately at a certain distance both the point and print will be equally distinct ; 

 the point and print mark the extreme limits of the line of accommodation. 



12. Astigmatism is usually due to unequal curvatures of the cornea 

 in different meridians, i.e.y the surface of the cornea is not part of 

 a perfect sphere. Astigmatism is not uncommon, and usually the 

 curvature of the cornea is greater in the vertical than in the hori- 

 zontal meridian. This is "regular astigmatism." In such a 

 " spoon-shaped " cornea a point of light is not focussed as a point — 

 " pin focus," but is linear or " line focus." 



(a.) Draw on a card two black lines of equal thickness, intersect- 

 ing each other at right angles. Fix it vertically at the far limit of 

 accommodation and look at it, when probably either the vertical or 

 the horizontal line will be seen more distinctly. Test each eye 

 separately. The line most distinct corresponds to the meridian of 

 least curvature of the cornea. 



{h.) Instead of a cross, construct a star, the lines radiating at equal angles 

 from the centre, and being of equal thickness. Repeat the previous observa- 

 tions, observing in which meridian the lines are most distinct. 



(c.) Repeat these observations with the "astigmatic clock" suspended on 

 the wall, or with ap})ropriate illustrations given in SnellnCs "Test-types." 



{(i.) Construct a series of concentric circles of equal thickness and tint, 

 about one-eighth of an inch apart upon a card. Make a small hole in the 

 centre of the card. Look steadily at the centre of the card held at some 



