50 BARKER : 



liest form in which it was possible to adjust the focus, this 

 being effected by a screw movement to draw the object toward 

 the lens against the counteracting pressure of a spring. 



With such an instrument this great Dutchman discovered 

 the red blood corpuscles and other minute forms with which 

 every histologist is now familiar ; but as a matter of fact it 

 will be shown that his high powers were not needed for the 

 demonstration of the formation of images by the corneules of 

 the arthropodal compound eye, though the focussing arrange- 

 ment was indispensable. 



It is well known, of course, that the tunica cornea of the 

 compound eye, or what we now simply term the cornea, is a 

 hard, firm tissue, composed, with few exceptions as to 

 number,* of thousands of distinct units termed corneules, or, 

 superficially considered, facets, which are usually, but not 

 invariably, hexagonal in outline and more or less convex both 

 posteriorly and anteriorly (Plate I, Fig. i ; Plate II, Figs. 7 

 and 9), but while many persons are familiar with the state- 

 ment that each of these corneules will form a distinct image 

 of external objects, the writer believes that few have ever 

 seen them, though satisfactory arrangements can be made 

 with little difficulty b}^ any possessor of a microscope. 



Remove the eye of an insect whose eyes have large facets, 

 the dragon-fl}' for instance, and dissect away all except the 

 cornea, carefully' cleaning the interior with a camel's hair 

 pencil. Take a small portion of this tissue and mount it 

 "dry" or in a medium of low refraction, the anterior or 

 outer face of the cornea being placed downward. On a circle 

 of tracing paper about three-quarters of an inch in diameter 

 draw a star, rosette, or other simple design one-fifth of an 

 inch or less in diameter, making it as black as possible for 

 the sake of contrast. Fasten this to a carrier of some sort, 

 having an aperture a trifle smaller than the diameter of the 



* Folsoni states that the worker ant of Eciton has at most a single 

 facet on each siile of the head. 



