Stonry—Limitation of Insect Vision. 229 
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 1’ (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 1’ 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 dis- 
tinguishing 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, since 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 of them 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 4u,' measuring from the middle of 
one to the middle of the next. This interval is about half the 
diameter of the red corpuscles 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; 
1The micron w is the millionth part of a metre. This is the same as the 
thousandth of a millimetre, or the 1/25400th of an inch. 
2 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 con- 
verge, and finally becomes a cone of 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 undeyiated rays very nearly pass through a certain point which is situated 
close behind the optic lens, and 14 centimetre in front of the middle of the retina. 
This is the point which is called the ‘‘ optical centre’’ of the eye. 
