318 Dr. G. J. Stoney on the Limits of Vision: 



of human blood, an object familiar to every microscope- 

 observer. Again, the " optical centre " * of the eye lies a 

 centimetre and a half in front of this part of the retina ; 

 and at this distance the interval between adjoining cones sub- 

 tends an angle of nearly 1'. Hence, in order that the images 

 of two points of light may fall on the corresponding parts 

 of different cones, the^r distance asunder must subtend an 

 angle of, or exceeding, 1' at the optical centre of the eyes ; 

 in other words, the interval between the objects in external 

 nature that are being examined must subtend this angle at 

 the eye. Thus we fail to see with the unassisted eye much 

 detail which is :evealed to us by the microscope. This happens 

 if at a distance of ten inches, the distance of most distinct 

 vision, the intervals at which these objects are spaced subterd 

 an angle of less than V. Such objects may, however, be seen 

 with optical aid, provided it is such that the little interval 

 subtends an angle exceeding V at the optical centre f of the 

 object-lens used in the microscope, a point which, with the 

 higher powers of the instrument, lies close to the object on 

 the stage. But beyond this limit, and therefore beyond the 

 reach of the microscope, there are still worlds of events in 

 nature which we can never see, although we may infer the 

 existence of some of them in other ways. 



We have found that the spacing of the cones in the fovea 

 lutea is competent to put a limit to the minuteness of the 

 detail that can be seen with the naked eye. Now, the small 

 s^ze of the pupil of the eye also, and independently, determines 

 srch a limit. Astronomers are familiar with the fact that 

 the image of a star (wdiich is virtually the image of a point 

 of light, since no telescope is competent to show the true disk 



* From each point of a visible object a cone of rays, starting from that 

 point as its apex, falls on the pupil. In passing through the eye this cone 

 of rays is made to converge, and finally becomes a cone uf rays advancing 

 towards that point of the retina where the image is formed. The apex 

 of the second cone is accordingly at this point. Most of the rays of the 

 first cone are bent in passing through the cornea and optic lens, and 

 advance in a new direction in the second cone. But there is one among 

 them, which, in the second cone, continues in the same direction, or at 

 least parallel to the direction which it had in the first cone. This ray is 

 called the undeviated ray. It is easily seen that there is one such ray in 

 the light coming from each point of the object. Now all the undeviated 

 rays very nearly pass through a certain point which is situated close 

 behind the optic lens, and 1^ centimetre in front of the middle of the 

 retina. This is the point which is called the " optical centre "of the eye. 



t The optical centre of the object-lens of a microscope is the point 

 where the "undeviated rays" cross (see last footnote). In compound 

 microscopes this point lies in or in front of the object-lens, and with high 

 powers is close to the object. 



