146 EXPERIMENTAL PHYSIOLOGY. 



Move the object further from the lens. There are now two 

 blurred images of the object. Cover the left hole and note 

 that one image disappears. 



Place the object at a distance from the lens less than the 

 first. Note that in this case also a double image is formed. 

 Again cover the left hole and see whether the image which dis- 

 appears is the same one as before. 



Draw diagrams of the formation of images by the lens when 



the object is (a) at a distance from which the rays are focussed 



on the screen, (b) at a greater distance, (c) at a less distance. 



Show whether the image in each case is real or virtual. 



Since the focal length of a lens varies inversely as its strength, 



being shorter the stronger the lens, it is usual to express the strength 



of a lens in terms of the reciprocal of its focal length. The standard 



is a lens which has a focal length of one metre and this is said to 



have a strength of one dioptre (D = ^r, where F = focal length, 



r 



D = strength in dioptres). 



Refraction in the Eye. 



If the rays from an object pass in their course through a succession of focussing 

 surfaces, a calculation of the position, size, etc., of the image which they finally 

 form can be made along the lines which we have indicated as long as the surfaces 

 are only two, or at most three, in number. When they are more than that the 

 problem becomes unwieldly. In such a case, however, it is often possible by 

 mathematical calculation to arrive at an ideal, or imaginary, single convex refract- 

 ing surface which will have approximately the same power as that of all the 

 surfaces together. For this the radii of curvature and the refractive indices of all 

 must be known and the surfaces must be "centred". Surfaces are said to be 

 centred when they all lie along the same principal axis. Light entering 

 the eye has to pass through a large number of refracting surfaces, 

 through cornea, aqueous humour, lens, and vitreous humour. Of these, 

 the cornea, the aqueous humour and the vitreous humour have refractive indices 

 which are much alike, all about equal to that of water (ju = 1.3). The lens is 

 denser and all parts of it have not the same composition. It is made up of more 

 or less concentric layers about a central core and the density increases from the 

 outer layers (ju = 1.40) to the core (// = !. 44). When they enter the cornea from 

 the air, light rays undergo the main part of the refraction which occurs in the eye, 

 because the densities of the two media are so different. The other con- 

 siderable refraction occurs on the passage of the rays through the denser substance 

 of the crystalline lens. In the complex system of the eye the refractive index 



