Visual Organs of Insects and Crustacea, 127 



same relation to external objects, they will, when both are 

 equally illuminated, produce only one sensation. But, for 

 the corresponding parts of the two retinae to receive the same 

 impression, it is necessary that these eyes be movable, without 

 which condition there would frequently be double vision. 

 This is the case (the mobility of the organ) with the eyes of 

 the vertebrated animals and of the cephalopode Mollusca. 



But, if these eyes are not movable, it must necessarily follow 

 that their retinae cannot bear exactly the same relation to 

 external objects. Eyes of this kind, whose fields of vision are 

 separate, cannot be convergent exteriorly ; for, in that case, 

 single objects, illuminating both eyes, would of course pro- 

 duce double vision. Eyes with fields of vision altogether 

 different are therefore necessarily immovable and divergent. 

 Such are the eyes of the ^rachnides, &c., and the stemmata 

 and compound eyes of insects, &c. A plurality of these eyes 

 does not increase the intensity, but simply the extent of the 

 field, of vision. 



2. In the first species of the organ of vision, the production 

 of the image upon the retina is the consequence of the reunion 

 of the rays regularly emitted from the same points of the ex- 

 ternal object. But it may be conceived that an image will 

 also be formed when those rays which fall perpendicularly 

 upon the sentient surface are alone admitted, whilst all other 

 rays, coming from the same point of emission, but approach- 

 ing the retina at other angles of incidence, are intercepted by 

 any organ whatever. If the rays given out perpendicularly 

 from each point of the external object alone impinge upon the 

 retina, it matters little how this is effected, the image of the 

 object will be then reproduced, indistinctly, it is true, but still 

 correctly as it regards the relations of its different parts to each 

 other. We perceive, also, that such a retina cannot be spread 

 out over a level surface, for in such case it could receive the 

 perpendicular rays of light only from few objects, and those 

 of the smallest possible size. The sentient surface or retina 

 of a visual organ of the second species must consequently be 

 spherical, so that the rays of the sphere may correspond to 

 external objects situated in the direction of the rays. 



The quantity of light coming perpendicularly will of course 

 be very small ; but still, an organ sensible to light will per- 

 ceive all the minute differences in the form of the image, in 

 the same manner as it occurs in an eye of the first species when 

 the pupil is diminished to a mere point : the light passing 

 through a pupil so diminished will be sufficient for the pro- 

 duction of images upon the retina, provided only that the 

 proper insulation of the different rays takes place. In an eye 



