with Special Reference to the Vision of Insects. 317 



Fie l. 



Millimetric Ruling. 



Section I. — Of Vision in general. 



As preliminary to the inquiry it is well to consider what 



are the causes that limit the amount of detail that can be seen 



by the instrumentality of eyes such as our own, the kind of 



eyes of which we know most. That tl ere is such a limit to 



human vision may be easily seen by placing a well-illuminated 



ruling of parallel lines at different 



distances from the eye of a person 



whose vision is good. Let us suppose 



black lines ruled, as in fig. 1, on a 



white surface at intervals of one 



millimetre from the middle of one 



line to the middle of the next. If 



an observer with keen vision views 



these from a distance of eleven or 



twelve feet, he is able barely to make 



out that they are a ruling ; beyond 



that distance, they seem one uniform 



grey surface, while from stations 



nearer to them he perceives the individual lines distinctly. 

 Now, at a distance of eleven feet a millimetre subtends an 

 angle of V (one minute) . Hence we learn from observation 

 that in order that two objects may be seen as two, they must, 



at least, subtend an angle of about V at the eye. If they 

 subtend a less angle than this they are seen as one object. 



Now there are three distinct causes, any one of which is by 

 itself competent to put a limit of this kind to our power of 

 distinguishing minute objects ; and in persons with the best 

 vision each of these three seems to put nearly the same limit 

 as the other two. This adjustment between them is, no doubt, 

 the result of development, s^nce any further improvement on 

 the lines of any one of these causes would be useless, unless it 

 were accompanied by a simultaneous improvement in both 

 the others. 



One cause is the spacing of the cones that occupy the 

 fovea lutea, into the small area of which about 7000 are 

 packed. The fovea lutea is that spot in the retina which 

 furnishes us with the exceptionally distinct vision which we 

 have in the middle of the field of view. The cones are here 

 without accompanying rods, and are at intervals of about 4//, *, 

 measuring from the middle of one to the middle of the next. 

 This interval is about half the diameter of the red corpuscles 



* The micron p is the millionth part of a metre. This is the same 

 as the thousandth of a millimetre, or the l/25400th of an inch. 



