NOTES AND MEMOKANDA. 183 



without materially altering the form of the individual " lenses," which 

 would impair the distinctness of the images produced by them. Judg- 

 ing from numerous specimens of his mounting which the writer has 

 had occasion to examine at different times, he can recommend Mr. 

 Barnett as an excellent preparer of these " eyes " (as well as of insects 

 in general, grouped diatoms, wood-sections, &c), and whose mounts 

 may be relied on. To obtain all the effects which can be got from 

 this beautiful object requires some patience, care, and skill. No in- 

 structions can supply the place of these qualities, but we will endea- 

 vour to make our hints as plain and practical as possible. We will 

 first consider the mode of operating by daylight, which is the easiest to 

 work with, and admits of the greatest variety of effects. That we may 

 see the images at all, we must know where to look for them, which is 

 on the very summit of the quasi-lenses, or possibly a little above. One 

 general rule will suffice for finding them with any " power " or ar- 

 rangement. First, focus down to the hexagonal framework, which is 

 the lowest part ; when this is distinct the raised corneules will be 

 utterly lost. Then rack slowly back, the projecting rounded corneules 

 will come into view as their reticulated " setting " fades away. Con- 

 tinue raeking-uj> till they in turn disappear in a general haze, which 

 will be near the point required. Now pass a steel pen or similar 

 object between the mirror and stage with its broad side presented to 

 both. With slight focussing, a perfect image of it, moving backward 

 and forward, will be seen in each of the so-called " facets " of the eye. 

 With the writer's own slide, viewed with a 1-inch objective and a large- 

 field ocular (a " Kelner " gives the finest effect with all such " show- 

 objects "), nearly 2000 separate images are seen at once well focussed. 

 With J-inck or ^-inch objectives the size and distinctness of the 

 images are greatly increased, but their number is, of course, dimi- 

 nished in proportion. Still even the ^-inch gives 150 ocelli in the 

 field. Either the plane or concave mirror may be employed, but the 

 latter is probably preferable. A capital and appropriate object with 

 the higher powers is a dead house-fly, stuck on a pin passiug through 

 a piece of cork, which may be held by a stiff wire attached to any 

 stand or support placed on the right-hand side of the mirror. The 

 nearer the object is brought to the stage, the larger its image will 

 appear, while approximation to the mirror diminishes the apparent 

 size. Further, by suitably directing the mirror, we can make window- 

 bars, sash-fastenings, blind-tassels, &c, appear in each ocellus, though 

 small and somewhat indistinct. The size and perfection of definition 

 of such images may be greatly increased (put in by composition) by 

 employing with a ^-inch a 1-inch objective as an achromatic con- 

 denser. By racking this up or down, the size or definition of the 

 image may be regulated to a nicety. A blind-tassel may thus be mag- 

 nified so as nearly to fill the whole area of the corneules, or be more 

 than half an inch in diameter, while each " strand " will be distinctly 

 seen, and even the mottling of a fly's wing will be plainly visible. If 

 the tassel be set swinging, the effect will be amusing, and if it be drawn 

 aside and an outstretched hand with the fingers in rapid movement be 

 put in its place, the appearance of 150 frantic extremities in simulta- 



