188 



KNOWLEDGE 



[August 1. 1898. 



closely packed facettes, of usually hexagonal contour and 

 convex surface. They are in some instances so numerous 

 as to occupy nearly the whole of the head. 



A section through the eye of the drone-fly, Eristalis 

 tenax, gives a general idea of the structure of this organ. 

 The cornea, hyaline in shape, is a modification of the tough 

 exo-skeleton of the insect, and like it consists of chitine. 

 Behind, and in the centre of each facette, is a cone of 

 transparent gelatinous matter, with its base towards the 

 cornea ; this cone, which functions as a crystalline lens, 

 terminates at its apex in a nervous filament. The filaments 

 so produced converge as they proceed, and finally fuse into 

 an optic nerve which connects them with the great nerve- 

 ganglion or brain. Each lens with its nervous filament is 

 isolated from those adjacent by the opaque pigment with 

 which the intervening space is filled, so that no light passes 

 into the eye except at that point directly under the centre 

 of each facette. The pigment matter is variously coloured, 

 and is the source of that sparkling appearance often seen 

 in the eyes of living insects when viewed by reflected light. 



We are at present concerned, however, with the cornea 

 and its tiny lenses, through which we are to obtain our 

 photograph. Each lens is a compound of two plano- 

 convex lenses, united at their plane surfaces. The corneal 

 mass does not follow any definite curve throughout, being 

 considerably flatter in the middle than at the margin. In 

 some of the larger dragon-flies parts of the eye may be 

 found nearly flat, and on that account will serve our 

 purpose the best. Having secured a specimen of the 

 common dragon-fly, LiheUxdn depresm, we can proceed to 

 dissect off the cornea. First remove the head, and embed 

 it in a cell of melted beeswax, so as to obtain a firm hold, 

 leaving one eye exposed. Take a two-edged scalpel, and 

 with the point of it make a series of stabs along the margin 

 of the cornea, going all round. The entire mass, like 

 a little dish with a quantity of pigment matter in it, 

 can then be lifted off. Next melt the beeswax, remove 

 the head, and float the separated cornea, with its contents 

 uppermost, upon the still fluid wax and allow the latter to 

 set. Pare a quill to the shape of a J pen, making the 

 point slightly rounded and thin, and with it scrape out the 

 contents of the eye ; the cornea, being extremely tough, 

 is not liable to injury from the point of the quill. Procure 

 a small camelhair pencil, cut down the hairs until only an 

 eighth of an inch long, and, having charged the stump with 

 turpentine, proceed to twirl it about in the " dish," so as to 

 remove the last trace of pigment. Examine under a pocket 

 lens, and, if clean, melt out the cornea, and with a pair of fine 

 scissors trim ofi'the turned-up edge of the " dish," retaining 

 only the flattest part of the bottom. Soak in turpentine 

 until any wax that adheres is dissolved, and you have the 

 lenses for your camera — but the disc is not flat. Select 

 the thinnest cover-glass you can get, clean thoroughly, 

 and place it upon the hot plate as for an ordinary balsam 

 mount. Melt a quantity of the oldest and hardest Canada 

 balsam obtainable, and upon the point of a needle apply 

 a small bead of it, not larger than a pin's head, to the 

 centre of the cover-glass. By the aid of the fine forceps 

 and the warm air, drive off the turpentine from the piece 

 of cornea, and then place it upon the cover-glass, concave 

 side down, with its centre over the bead of balsam. Cut 

 out a small disc of notepaper, rather less in diameter than 

 the " object " you are mounting, and place it centrally 

 over the disc of cornea. Have a three-by-one glass slip 

 handy, support one end of it upon the hot-plate, and allow 

 the other to press upon the object, with the disc of paper 

 in between. Proceed to load the middle of the three-by- 

 one slip with coins sufiicient to flatten the piece of cornea. 

 As soon as this takes place the bead of balsam under it 



will spread out and make its appearance round the margin 

 of the object. 



When this can be seen through the slip, remove the 

 spirit lamp and allow the whole to cool. The under side 

 of the flattened cornea wiU then be cemented to the cover- 

 glass and the " facettes " will be simple plano-convex lenses, 

 since their inner convexities, being embedded in the balsam, 

 will cease to exert their optical properties to any appreciable 

 extent. The mount, resting upon a three-by-one slip, can 

 now be examined under the microscope. Put in a half- 

 inch objective, and, if it has a screw collar, adjust for an 

 " uncovered " object. Use a low power eyepiece of about 

 three diameters and an Abbe condenser. Stop down the 

 aperture and bring the cornea into sharp focus. By a 

 solid cone of transmitted light an hexagonal figure will 

 be seen forming the boundary of each facette. A number 

 of hairs appear upon the eyes of some insects, and it is in 

 the angles of these hexagons that the hairs have their 

 origin. It will require some nicety of illumination, how- 

 ever, to disclose the "lens" in each of these divisions; 

 the solid cone of light as usually transmitted by the 

 condenser fails to reveal them. Remove the limiting 

 diaphragm from the back of the condenser and insert in 

 its place a patch-stop that will give dark-ground Illumination 

 with the objective employed, having previously stopped 

 out half the annulus with a strip of black paper. Use the 

 plane mirror, and, with the condenser in focus, this semi- 

 annular beam of light will fall obliquely upon the object 

 and the " eye-lenses ' will appear in bold relief (Fig. 1). 



yyi'u 



Fig. 1. — Group of '■ Eyp-lenses " ready for tlie Camera. ( x 200.) 



To prepare the mount for the microscope, select a three-by- 

 one sUp of wood, through the centre of which a small hole 

 has been bored. Lay the cover-glass with the object face 

 downwards over the hole and fasten with a punched gummed 

 label, after the manner of a French paper-mounted slide. 

 Fix the shp firmly iu the stage of the microscope. The 

 cornea, being under the cover-glass, is protected from 

 injury, and the facettes, facing the condenser, are in the 

 right position for our experiment. 



Next incline the microscope to a horizontal position, and 

 fix it in its place upon the baseboard of the camera. 

 Correct the objective to the thickness of the cover-glass, 

 remove the mirror, and the diaphragms and stops from 

 the substage condenser. Place a microscope lamp about 

 ten inches from the condenser, with its flame in the optic 

 axis of the apparatus and the bull's-eye turned aside. 

 Withdraw the camera /'Co tem., and proceed to focus up 

 the middle of the object. When the field is full of sharply 

 defined hexagons, and an image of the lamp-flame is seen 



