SCHEINER'S EXPERIMENT. 



285 



(f.) Place the removed lens in front of the cornea, the 

 principal focus is now much in front of the retina, so that 

 a much weaker lens than the one removed has to be used 

 after removal of the lens for cataract. 



(g.) Astigmatism. Fill the plano-convex glass (g) to 

 imitate a cylindrical lens with water, and place it in front 

 of the cornea. Between the cornea and the cylindrical lens 

 place a sheet of zinc with a cross cut out in it, or with a 

 number of holes in a horizontal line. One cannot obtain a 

 distinct image of the cross or the holes, as the case may be. 



Fig. 137. Kiihne's Artificial Eye, as made by Jung of Heidelberg. 



(h.) Schemer's Experiment. With the light properly 

 adjusted, place in front of the cornea a piece of zinc per- 

 forated with two holes (c), 1 cm. in diameter, in a horizontal 

 line, the distance between the holes being less than the 

 diameter of the pupil. Find the position of the retina and 



