LXXIV.] 



kDhne's artificial eye. 



351 



(c.) Place convex and concave lenses between the source of liglit and the 

 cornea ; observe how each alters the course of the rays and their focus. 



{d.) After having an image well focussed upon the retina, move the latter 

 away from the lens, when the image becomes blurred owing to diffusion. If, 

 however, a slip of zinc, with a hole cut in it to act as a diaphragm to cut oif 

 some of the marginal rays, be interposed, the image is somewhat improved. 



{e.) After seeing that the light is sharply focussed on the retina, remove the 

 lens— to imitate the condition after removal of the lens for cataract— and 

 observe that the rays are focussed quite behind the retina. 



(/.) 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 hns to be used after removal of the lens for cataract. 



Fig. 27a.— Kuhne's Artificial Eye, as made by Jung of Heidelberg 



{g.) Astigmatism.— Fill the plano-convex glass (^r)— 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. 



(Ji.) Scheiner's Experiment.— With the light properly adjusted, place in 

 front of the cornea a i)iece of zinc perforated with two holes {c), i 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 there is only 

 one position— in which the two beams of light are brought to a focus. Move 

 the retina towards the cornea, and observe two images ; close the right-hand 



