270 EXPERIMENTAL PHYSIOLOGY. 



to the corneal one, while the other two undergo no per- 

 ceptible change. At the same time the pupil becomes 

 smaller. 



10. Line of Accommodation i.e., the eye does not accommodate 

 for a point, but for a series of points, all of which 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 in 

 a blackened card, about 2 mm. apart, 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 focussed 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, we can see at one and the same time only one 

 of the objects ; but not the point, and the print equally 

 sharply defined. Remove the eye gradually from the glass 

 plate, and ultimately at a certain distance both the point 

 and print will be equally distinct, and the point and print 

 mark the extreme limits of the line of accommodation. 



11. Astigmatism is usually due to unequal curvatures of the 

 cornea in different meridians. 



(a.) Draw on the card supplied to you two black lines of 

 equal thickness, intersecting 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 cur- 

 vature of the cornea. 



(6.) Instead of a cross construct a star, the lines radiating 

 at equal angles from the centre, and being of equal thickness. 

 Repeat the previous observations, observing in which meri- 

 dian the lines are most distinct. 



(c.) Repeat these observations with the " astigmatic 



