392 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



appearance of ovoid spindles, each enclosed in a cylindrical sheath. The spindle-shaped 

 bodies underwent rapid change of form from the escape of their contents, so that in a 

 short time they were reduced to the condition of empty shrivelled tubes. 



Further observations on various insects have convinced me that such changes usually 

 occur shortly after death, either from osmosis or from alterations of tension. 



These observations suggested to me the possibility that the spindles of the great rods 

 should be regarded as lenses of very short focal length, but of great thickness, and that 

 they form a second refractive system, the anterior foci of which correspond with the 

 subcorneal images, and the posterior foci with the bacillar layer of the retina. 



According to this view the dioptron is composed of an anterior and a posterior refrac- 

 tive system. 



The anterior refractive system of each segment of the dioptron consists of a corneal 

 facet, a lenticulus, to be hereafter described, and the anterior face of the crystalline cone. 



These parts form a subcorneal image, which lies in the anterior focus of the posterior 

 refractive system. 



The posterior refractive system magnifies the subcorneal image, and erects it, at its 

 posterior focal plane, upon the bacilla of the neuron. 



It is well known that if an objective is placed in the reversed position beneath the 

 stage of a microscope, and the instrument is focussed for its posterior focal plane, it can 

 be used as a telescope. A segment of the dioptron of an arthropod's eye, according to 

 my view, is comparable to such an instrument, and the whole dioptron to as many 

 instruments as there are segments, each giving a perfect picture of the objects which 

 subtend a small angle with its axis ; and thus a mosaic of images, which are erect and not 

 reversed, falls upon the retina. 



Although the ordinary simple eyes of insects do not exhibit any structure comparable 

 to the second refractive system of the dioptron, the simple eyes of many larval insects 

 have a posterior refractive system, and afford a complete transition from a simple to a 

 compound eye. 



I shall now proceed to the consideration of the details of structure and measurement 

 on which my theory is based, in the following order : — 



I. The Anatomy and Functions of the Dioptron. 

 II. The Anatomy and Functions of the Neuron. 



III. The Development of the Compound Eye. 



IV. The Morphology of the Eyes of Arthropods. 



I. The Anatomy and Functions of the Dioptron. 



The dioptron in the most highly specialized forms of compound eye, such as are found 

 in the majority of perfect insects, is entirely enclosed in a chitinous case, formed by the 

 compound cornea and the basilar membrane, united to each other by an inflection of 

 the integument, which forms a short hollow cone between them. 



The cornea and the basilar membrane are nearly parallel surfaces, so that the whole 



