336 PRACTICAL PHYSIOLOGY. [LXXI. 



distance. All the parts will not be equally distinct. Approach the card 

 towards you, noting in which diameter the lines appear most distinct. 



(e.) This card may be used in another way. Hold the card in front of, and 

 with the circles directed towards the eye of another person especially one 

 with astigmatism ; place your own eye behind the hole in the card and look 

 into the observed eye, noting the reflection of the circles to be seen in the eye. 

 Observe in which meridian the circles are most distinct, and if there be any 

 perceptible difference in the thickness and distinctness of the circles. 



(/.) Draw a series of parallel, vertical, and horizontal lines of equal tint and 

 thickness, and about one-eighth of an inch apart. Fix the card vertically at 

 a distance, and move towards it, noting whether the vertical or horizontal 

 lines are most distinct. 



(#.) Fix a fine wire or needle vertically in a piece of wood moving in a slot, 

 and similarly fix another needle or wire horizontally. Move the needles 

 until both can be seen distinctly at the same time, when it will be found that 

 the needles are some distance apart ; usually the horizontal one is the nearer. 



13. Diplopia Monophthalmica. 



Make a small hole in a black card, hold it at some distance, and with one eye 

 look through it at a luminous point, the eye being accommodated for a distant 

 object. One sees either several objects (feeble light) or an irregular radiate 

 figure with four or eight rays. Move the paper, and the long rays remain in 

 the same position. Compare the figure obtained from the other eye. It will 

 very likely be different. 



14. Movements of Iris. (i.) It is an extremely beautiful experi- 

 ment, and one that can easily be made by looking at the white shade 

 of an ordinary reading-lamp, to look through a pin-hole in a card at a 

 uniform white surface. With the right eye look through the pin- 

 hole, the left eye being closed. Note the size of the (slightly dull) 

 circular visual field. Open the left eye, the field becomes brighter 

 and smaller (contraction of pupil), close the left eye, after an 

 appreciable time, the field (now slightly dull) is seen gradually to 

 expand. One can thus see and observe the rate of movements of 

 one's own iris. 



(ii.) Pupil-Reflex. 



Place a person in front of a bright light opposite a window, and 

 let him look at the light, or place oneself opposite a well-illuminated 

 mirror. Close one eye with the hand and observe the diameter of 

 the other pupil. Then suddenly remove the hand from the closed 

 eyo, light falls upon it; at the same time, the pupil of the other eye 

 contracts. 



15. Pupil of Albino Rabbit. The pupil in albinos appears red, 

 although in other animals it is black. In the albino it is red owing 

 to the absence of pigment in the choroid and iris, so that light is 

 admitted through the sclerotic and choroid and is reflected from 

 the interior of the eyeball through the pupil to the eye of the 

 observer. 



