SANS0N-PUI1K1NJE IMAGES 219 



formed, i.e. the surfaces separating media of different refrangibility, 

 are (1) the anterior surface of the cornea, (2) the anterior surface of 

 the lens, and (3) the posterior surface of the lens. We shall find 

 that reflected images are formed by these three surfaces and that 

 the first produces the brightest image because it separates two media 

 possessing a greater difference of refrangibility than is the case with 

 the other surfaces. The clear image already described comes, then, 

 from the anterior surface of the cornea. On examining carefully in a 

 darkened room, a second image will be observed lying apparently be- 

 hind the first, much less bright than that image, but erect and somewhat 

 larger than the first image. This image is therefore produced by a 

 convex surface lying behind, and less curved than the anterior sur- 

 face of the cornea. It is from the anterior surface of the lens. On 

 further examination a third image can be made out less bright than 

 either of the preceding ; it is, moreover, inverted, real, and smaller 

 than either. It comes, therefore, from a concave surface more curved 

 than either of the preceding. This surface is the posterior surface 

 of the lens. These three images are called the Sanson-Purkinje 

 images. The changes in size and position of these images may be 

 utilised to prove that the curvature of the lens alters during accom- 

 modation. For this purpose the observed eye shovfld first be accom- 

 modated for a distant object and then for a near one, the reflected 

 images being observed as the change occurs. It will be found that 

 the first image remains unchanged, but that the second image becomes 

 smaller and moves nearer to the first. This proves that the surface 

 forming it, the anterior surface of the lens, becomes more convex. 

 The movement towards the first image is due to two causes : firstly, 

 that the anterior surface of the lens becomes more convex ; and 

 secondly, that the anterior surface of the lens approaches a little 

 nearer to the cornea. 



The third image is also found to change, but to a much less degree. 

 It becomes a little larger, and appears to move further from the 

 second. The apparent size and position of this image are, however, 

 modified by the fact that it is viewed through the two surfaces of the 

 cornea and lens ; and careful measurements of this image have shown 

 that the changes observed are due, not to a change of curvature of the 

 posterior surface of the lens, but to the change in the anterior surface 

 through which it is viewed. 



The observation and measurement of these images are much facilitated 

 by the phakoscope, an instrument devised by von Hehnholtz for that pur- 

 pose. It consists (rig. 171) of a triangular box whose angles are cut off. At one 

 angle two prisms, b and b' ', are fixed which, when illuminated, concentrate two 

 beams of light upon the observed eye. At the opposite angle, ". is an aperture 



