LXXI.] ACCOMMODATION. 331 



On continuing to look at the hearts, it will appear to move or flutter over 

 the blue background. 



(ii.) On a bright blue ground make a square with black lines and subdivide 

 it into smaller squares. On the same ground make a series of small squares — 

 not coinciding with the previous ones— with red boundaries. On moving 

 the figure to and fro in the shade below the level of one's eyes, one sees the red 

 squares moving to and fro over the black ones. Some see the black moving 

 behind the red. (" Zur Erklarung d. flatternden Herzen," A. Szili, Da Hois 

 Archlv, 1891, p. 157.) 



5. Accommodation. 



(a.) Standing near a source of light, close one eye, hold up both 

 forefingers not quite in a line, keeping one fiiiger about six or seven 

 inches from the other eye, and the other forefinger about sixteen 

 to eighteen inches from' the eye. Look at the Uf^ar finger ; a 

 distinct image is obtained of it, while the far one is blurred or 

 indistinct. Look at the far image ; it becomes distinct, while the 

 near one becomes blurred. Observe that in accommodating for the 

 near object one is conscious of a distinct effort. 



(b.) Ask some one to note the diameter of your pupil Avhen you 

 accommodate for the near and distant object respectively. In the 

 former case the pupil contracts, in the latter it dilates. Ask a 

 person to accommodate for a distant object, and look at his eye 

 from the side and somewhat from behind ; the half of the pupil 

 projects beyond the margin of the cornea. When he looks at a 

 near object in the same line, and without moving the eyeball, 

 observe that the whole pupil and a part of the iris next the observer 

 are projected forwards, owing to the increased curvature of the 

 anterior surface of the lens. 



(c.) Hold a thin wooden rod or pencil about a foot from the eyes, 

 and look at a distant object. Note that the object appears double. 

 Close the right eye ; the left image disappears, and nice versd. 



{(l.) At a distance of six inches from the eyes hold a veil or thin gauze in 

 front of some printed matter placed at a distance of two feet or thereby. Close 

 one eye, and with the other one soon sees either the letters distinctly or the 

 fine threads of the veil, but one cannot see both equally distinct at the same 

 time. The eye, therefore, can form a distinct image of a near or distant object, 

 but not of both at the same time ; hence the necessity for accommodation. 



6. Scheiner's Experiment (fig. 249). 



(a.) Prick two smooth holes in a card at a distance from each 

 other less than the diameter of the pupil. Fix two long fine 

 needles or straws in two pieces of wood or cork. Fix the card- 

 board in a piece of wood with a groove made in it with a fine saw, 

 and see that the holes are horizontal. Place the needles in line 

 with the holes, the one about eight inches and the other about 

 eighteen inches from the card. 



