578 EIIRENBERG ON THE POWER OF VISIOX OF THE EYE 



If there are several near to eacli other or several rows, they make a 

 joint impression on our eye and deceive us, as if we saw a greater 

 simple body or surface*. The general distance which good eyes main- 

 tain when striving to recognise these minute bodies, I found by mea- 

 surement to be from four to five inches, often six inches ; the latter is 

 however the general distance for very sharp-sighted persons. Myopical 

 persons seldom bring the same objects to within four inches, and still 

 more rarely to three, &c., and become then for the most part like others. 

 Any one whose distance of clearest vision is four inches cannot by a 

 greater approaching of the eye to the object increase his power of vision, 

 but feels pain, and does not see distinctly. If we have once hit on the 

 object, we may remove it considerably without losing sight of it. I 

 myself cannot see -^^ of a line at twelve inches' distance black on white; 

 but having found it at from four to five inches' distance, I can remove 

 it to twelve inches and still see the object plainly. This phasnomenon 

 is founded on the known power of the eye to accommodate itself some- 

 what to distance; we can often discern small objects at a greater di- 

 stance when our attention is directed to the place, or when they move. 

 Similar phsenomena are found in the air-balloon in a clear sky, and a 

 ship in the horizon ; we easily see them after they have been pointed 

 out to us, but the faculty of rapidly descrying depends on custom and 

 on the acuteness of sight, without permitting of any conclusion as to 

 tlie power of vision in general. When any person is more strongly ex- 

 cited by visual impressions than another he discerns them quicker, but 

 he does not on that account see more than the other, who because he 

 does not receive these impressions so vividly discovers them slower. I 

 often employ a glass to seek for objects which I wish to examine 

 with the naked eye, in order to give them another position with a 

 pointed instrument; this fact has relation only to finding the place of a 

 body, and merely furthers quickness of discernment. Myopical eyes always 

 find out things more easily, because they are less separated, and their field 

 of vision is smaller. Probably there is still a higher degree of the ab- 

 solute power of vision of the human eye to add, which is that of the 

 discerning of luminous bodies. In the dark, small luminous bodies 

 appear, as is known, much larger than they are; and these, whether they 

 be themselves luminous or only reflectors of light, are capable ac- 

 cording to the strength of the light of easily affecting the human eye 

 at a much less magnitude than the ^jj of a line. I have never had the 

 opportunity of observing self-lighting magnitudes which were really of 

 so small a diameter that I could direct attention to a limit in regard 



* I am accustomed in this manner to discern very fine cilia of the infusoria. 

 As soon as they are moved they form a small apparent surface, which is percep- 

 tible ; as soon however as they leave off, their lineness is often such that the 

 power of vision does not reach them with the microscope. 



