20 METHODS OF MICROSCOPICAL RESEARCH. 



blind, it cannot perceive a trace of light, and if 

 the image of an object falls upon this blind spot, 

 that object is totally invisible e It is at this spot 

 also where the blood-vessels enter the eye, and 

 ramify through nearly the whole of the surface 

 layers of the retina. 



In the above description points only have been 

 touched which directly bear on good or defective 

 vision. On the other hand, enough has been 

 advanced to show that this organ is liable to im- 

 perfections which may be, and are, extremely liable 

 to modify all our observations made over the tube 

 of the microscope. 



Now, if we take an ordinary lens of glass and 

 attempt to produce a picture with it, we find the 

 centre alone is plainly visible the lens is afflicted 

 with what is termed spherical aberration, that is, 

 the rays from its periphery are brought to a focus 

 in a different plane to those occupying a central 

 position. 



But a small amount of spherical aberration is not 

 readily detected by the student. It appears as a 

 haze or fog of light over the object. 



In the human eye this defect is not observable to 

 any great degree, as the peripheral or more strongly 

 refracting rays are cut off by the iris. Then, again, 

 the curvature of the cornea is ellipsoidal rather than 

 circular, so that the rajs farthest from the axis are 

 least deviated, while the two curves of the crystal- 

 line lens correct, so to speak, the one the other ; 

 and lastly, this lens is of such construction that its 

 refractive power diminishes from the centre to the 

 circumference. 



Another defect in the eye is due to the different 

 meridians having dissimilar degrees of curvature. 



