b R. P. WILLIAMS ON 



would be afc the same time capable of being cut into very thin slices 

 without breaking. Such a wax I at length obtained, by melting 

 togelher butter of cocoa and bleached bees-wax, adding a little 

 new Canada balsam. Butter of cacao, extracted from the seeds of 

 the Theobroma cacao, melts, according to Thompson, in his 

 " Chemistry of Organic Bodies," at 122°, and is capable of being 

 rendered almost colourless by agitation in hot water. I could 

 not succeed in doing so with the samples I obtained, which also had 

 very variable melting points ; one sample melting at 80°, and 

 another as high as 100°. However, a compound wax with any 

 desired melting point can be readily prepared in the manner just 

 mentioned. That which melts at about 120° can be safely recom- 

 mended 



Let the bead be put up in the wax in such a way that the 

 cutting instrument shall be presented at right angles to the chord 

 of the more or less circular segment, which is the figure of the 

 outline of the eye ; thus, the plane of the section' will pass through 

 the centre of figure of the eye. 



By far the most satisfactory section to take, is that cut in a 

 direction to show the structure of both eyes,* as sometimes a feature 

 not observable in the one may be discovered in the other. It may 

 at first sight appear difficult to imagine how this can be the case, 

 and, indeed, it has been questioned if such a section can be cut. 

 It will, I am sure, be evident that it can be done, when we reflect, 

 that if a plane passes through a line joining the centres of two 

 contiguous spherical segments, that it must — it would appear 

 almost superfluous to add — be radially presented. If the head, 

 having been put up in the wax, be tilted up a little at the crown, 

 such sections may be removed, when the cutter passes in the 

 direction indicated, i.e., through a line joining the centres of 

 figure for each eye, and only such sections will show the radial 

 arrangement of the vessels, whatever position the head occupies 

 whether cutting for one eye or for both. The razor or cutter 

 being presumably perfect, must be moistened with spirits of 



* These sections, in the case of the Blow and House Flies, will show, when well 

 cut, not only the structure of the compound eyes, but also the very curious mem- 

 brane named by Mr. Lowne, the "Frontal sac," and which is said by him to be 

 an auxiliary organ of special sense — that of the olfactory. It is suspended almost 

 free in the frontal cavity, being attached but slenderly to the integument in front 

 and posteriorly to the central ganglion of the head. (See ," The Anatomy of the 

 Blow Fly," pp. 40 and 94.) 



