498 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the closely apposed elements present. The actual distinctness of the 

 features of the object is less important here than with the human eye. 

 The reason for the existence of two forms of eye, simple and com- 

 pound, in perfect insects, is that of separating the impressions of 

 space and distance from those of distinct sight of the object (the 

 latter end being attained by the stemmata or simple eyes). Four, on 

 the whole strongly distinct, kinds of vision are differentiated in the 

 animal kingdom : — 



1. General sensation of the amount of light evolved in the environ- 

 ment and of the relative position of its source ; analogous to our sensa- 

 tion of warmth, and exhibited only in small organisms with trans- 

 parent outer coverings, and devoid of special portions of the body 

 adapted for the function. 2. Sensitiveness to colour and shades of 

 colour ; general orientation as to environment, power of recognizing 

 known objects— the "eye-spots" of Vermes, &c. 3. Information 

 as to relative positions of surrounding objects affording guidance of 

 movements, with slight amount of guidance as to cbaracters of 

 object — compound eye of ArtJiropoda. 4. The most clear and faith- 

 ful perception of the objects, the images reversed by a lens which 

 strongly refracts light. The contents of a plane are the subject of 

 this kind of vision, which does not convey to the brain the distance or 

 mutual relations of objects ; the plane may be either single, at a constant 

 distance from the eye, or there may be several at distances which 

 vary within certain limits, as when accommodation comes into play. 

 In this case the third dimension, viz. depth or distance of objects, 

 is obtained by movements made by the eye-bearing individual, 

 relatively to the objects viewed, materially aided by the power of 

 accommodation, when this is present ; in its absence, as in the case 

 of the stemmata of Arthropoda, this impression must be very feeble, 

 since the moving animal obtains nothing but a disconnected series of 

 images of the objects as they come one by one within the range of its 

 organs. 



The phylogeny of the compound eye is deducible from the fact of 

 the acquired character of the movements of Arthropoda; as the 

 faculty of motion became better developed, the organs of sight 

 became modified, pari passu, into that form which successfully met 

 the requirements of this mode of motion, in the manner above 

 explained; the highest degree of development being naturally 

 reached in the Insecta. All winged insects are thus provided, while 

 but few of the wingless forms, such as larvas, &c, have this form 

 of eye. 



The explanation of the peculiar character of the vision enjoyed by 

 the compound eye lies in the lenticular curvature of the corneal facets, 

 which do not act as Joh. Miiller supposes, by magnifying the entering 

 rays, but by admitting only those which are not likely to prove 

 injurious ; this appears to be shown by a comparison of the Insect 

 with the Crustacean eye. The latter is remarkable (judging by the 

 results obtained by Grenacher from Mysis) for its large angle of 

 vision — 3° 16' in the instance taken — and is probably fitted to convey 

 impressions from a distance not exceeding a metre. Taking into 



