570 THE SENSES AND SENSORY ORCANS. 
antero-posterior diameter of the eyeball, the distinctness of 
vision if this hypothesis were correct would diminish with the 
size of the eye. Now, the evidence goes to prove that birds 
see quite as distinctly as we do; a Hawk with an eyeball having 
an antero-posterior diameter of 5 or 6 mm. should not see 
objects as distinct which subtend an angle less than 3' or 4’, 
whilst in man the estimated angle is less than 1’ (59” is usually 
given). It is therefore probable that each retinal element in 
such birds produces a complex sensation, and the same may be 
and probably is, true in insects. So that although on the above 
hypothesis an insect such as the Blow-fly appears to have only 
zy Of the acuity of vision which man possesses, if it were com- 
pared with a bird, it is possible that +, or even }, would bea 
nearer estimate ; and it must be remembered that Dragon-flies 
and Hymenoptera have vision which is many times sharper 
than that of the Blow-fly when calculated on the same 
method. My own estimate of the vision of the Dragon-flies 
(42schna) and Wasps (Vespa) is that they have seven or eight 
times the acuity of vision possessed by the Fly, and this would 
equal that of the Hawk or Swallow—a conclusion which is, I 
think, borne out by the experience of those who have watched 
these insects in a state of nature. 
The Illumination of the Retinal Image.—Every microscopist is 
aware that the higher the magnifying power and the smaller the 
front lens of the objective the greater is the need to increase the 
illumination. It will be manifest that the smallness of the corneal 
lenses necessitates a brilliant light, if, as I have maintained, a 
real image is formed at the inner extremity of the great rods. 
As a matter of fact most insects require a brilliant light, and 
are only active when the sun is shining. Many insects, as the 
Noctuids and Sphingidz, are, however, remarkable exceptions. 
In these the large size of the great rods is conspicuous, and 
I have observed the same thing in all nocturnal insects, and 
especially in the Tipulidz and Gnats. 
Many nocturnal insects are remarkable for the large size of 
the corneal facets, and the great reduction in the number of 
the ommatea in the compound eye. There is a marked con- 
