ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 727 



observation, because the simultaneous fulfilment of tbe following five 

 conditions is demanded ; — (1) The plates must be perfectly parallel, 

 have perfectly smooth surfaces, and be composed of quite pure 

 material. (2) The plates must be absolutely vertical to one optic 

 axis. (3) Must be for one kind of light only. (4) The incident 

 light must consist of parallel rays. (5) The Microscope must be 

 absolutely free from defects. As conditions 1 and 2 are only 

 occasionally, and 3, 4, 5 never, fulfilled, theory and practice give 

 contradictory results. Frequently, when thin-ground, section surfaces 

 may be found which remain uniformly clear, and without the appear- 

 ance of interference colours when examined with crossed nicols. 

 This property of remaining clear between crossed nicols depends on 

 the phenomenon of internal conical refraction. The author then 

 shows how, by means of plates of bichromate of potash, this internal 

 conical refraction can be studied. A plate of this salt is fixed with 

 wax to a rod, and the rod fastened in such a manner that the optic axis 

 lies in the centre of the field of vision. Instead of the lower nicol, 

 a very small diaphragm is inserted. Over the diaphragm a strip of 

 tinfoil, perforated by a tiny hole, is placed, so that the hole lies in the 

 -centre of the visual field. The Microscope is then pushed under the 

 bichromate plate, and the diaphragm raised until it is quite close to 

 the plate. The Microscope is fitted with a weak objective and a 

 strong ocular. At a certain focus, instead of the round hole, a bright 

 ring is perceived. The light of this ring is polarized, as may be 

 proved by placing a nicol on the ocular. Hence the author shows 

 that, in spite of theory, a plate cut vertically to one optic axis is 

 always bright between crossed nicols. The internal conical refraction 

 was also examined in topaz, andalusite, staurolith, adular, diopsid, 

 epidote, and arragonite. 



Enock's Sketches. — Under this title Mr. F. Enock is issuing 

 lithographic illustrations of some of his slides, the various parts being 

 numbered and named. In addition a short explanation is given, the 

 following being that accompanying sketch No. 3 — the head of a 

 ground-bee : — 



" This bee belongs to Section 2 of the British Aculeate Hymen- 

 optera, in which the hairs on the body, &c , are more or less branched 

 or plumose, especially those on the legs of the present example, 

 Colletes Daviesana. 



The tongue (10) is short and bifid, a good type of the Obtusilingues. 



The labial palpi (11) are hidden away under the lingua (10), 

 and cannot be seen from the upper side. The paraglossa (8) are two 

 small organs, having a few strong hairs on the margin, situate on the 

 upper side, and at the base of the lingua (10). 



These bees burrow in the sand, using their mandibles (5) for this 

 purpose, and wear the tips quite blunt by the time they have com- 

 pleted their work. 



This head is specially prepared for the paraboloid, but by care- 

 fully illuminating with the 'silver side reflector,' the puncturation 

 on the face, &c, can be well brought out." 



