HARDWICKE'S SCI ENCE-GOS SIP. 



105 



prism is used, the zero will be at the point where 

 the light is excluded. 



When we wish to obtain the value of the arc of 

 rotation produced by a liquid, it is necessary to 

 employ homogeneous light, and for this purpose red 

 light is preferred, as it is the only homogeneous 

 light that can be obtained by coloured glass : a 

 disc of red glass is therefore placed between the 

 eye and the analyzer. 



If the liquid in the tube has the property of pro- 

 ducing rotatory polarization, the secondary image 

 becomes evident when a double-image prism is 

 used. The analyzer should now be turned to the 

 right or left (as the case may be) until the image 

 disappears : the space traversed by the index, from 

 the zero indicates the arc of rotation. With a 

 Nichol's the light will be no longer excluded 

 at the zero, and the index must be turned until 

 the ray disappears ; the distance traversed by the 



index showing the arc of rotation. 



F. KiTTON. 



THE " CHALLENGER " EXPEDITION. 



-pUOFESSOR WYVILLE THOMSON has 

 -*- recently communicated to Nature an iateresting 

 description of the results of their dredgings, from 

 which it will be seen that this expedition is already 

 beginning to yield rich zoological fruit. Several 

 dredgings off the Portuguese coast were rich in 

 the usual Atlantic ooze, but little else. After trying 

 the ordinary trawl, the naturalists were fortunate 

 enough in securing, off Cape St. Vincent, and at a 

 depth of 600 fathoms, several new species of fish. 

 Two of them belonged to the genus Macrourus, 

 whilst another form was quite unknown to them, 

 atlthough approaching in many respects the genus 

 Mugil. All the fishes were in a peculiar condition, 

 from the expansion of the air in their bodies. The 

 Professor goes on to say : — 



" After this attempt we tried the trawl several 

 times at depths of 1,090, 1,525, and finally 2,125 

 fathoms, and always with success. 



"Several fishes, most of them allied to Macrourus, 

 were added to the list. Several decomposed crus- 

 taceans, and among the lower crustaceans at 1,090 

 fathoms a gigantic amphipod, of the family Hyperina^ 

 allied to Phronlma. The eyes of this creature are 

 very remarkable, extending in two great facetted 

 lobes over the whole of the anterior part of the 

 cephalo thorax, like the eyes of an Aeglina among 

 Trilobites. This crustacean, which is three and a 

 half inches in length, makes a splendid drawing, and 

 reminds one of the old Eurypterids, is in process 

 of description at the hands of Dr. von Willenioes 

 Suhm. 



" Mollusca are very scarce in deep water, and our 

 catches have hitherto been chiefly confined to such 

 things as the species of Nvctdu, Leda, Ferticordkt, 



&c., familiar through the deep dredgings of the 

 Porcupine. Among the molluscoids a haul in 1,525 

 fathoms gave us a lovely thing, a bryozoon, forming, 

 out of branches closely resembling those of Accro- 

 marchis neritina, a graceful cup, the bases of the 

 branches united by a transparent stem between two 

 and three inches high, like the barrel of a quill, or 

 the stem of a claret-glass. This genus, which pre- 

 sents a general character totally different from any- 

 thing hitherto known among recent Bryozoa, I mean 

 to dedicate to Captain Nares, as an early recogni- 

 tion of the confidence and esteem which he has 

 already fully!gained from the scientific staff. Naresia 

 cyathus certainly recalls, in a most singular way, 

 the Cambrian Dictyonema, a form which I had, how- 

 ever, hitherto been inclined to refer to the Hydrozoa. 



"The Echinoderms have yielded some exceedingly 

 interesting species to the trawl ; among them several 

 examples of the beautiful little urchins, of which 

 one specimen was taken by Count Pourtales, in 

 the Straits of Florida, aud described by Alexander 

 Agassiz under the name of Salenia varispina. It is 

 undoubtedly a true Salenia, and to an advocate of 

 the doctrine of the ' continuity of the chalk,' it 

 is pleasant to see in the flesh thig little beauty, 

 which has hitherto been reckoned among the lost 

 tribes. 



" Among the star-fishes two species of the genus 

 Hymeiiaster have occurred, and the ophiurids arc 

 well represented, chiefly by large examples of several 

 species of the genus Ophiomusium.^ 



"All the hauls of the.trawl, down to 2,125 fathoms, 

 have yielded many specimens of a singular Holo- 

 thurid, of which a description will shortly be pub" 

 lished by Mr. Moseley. The animal is of a rich 

 violet-colour. Like Psolus, it has a distinct ambu- 

 lating surface, with a central double line of water- 

 feet. The body cavity is small, but the perisome is 

 represented by au enormously thick layer of jelly, 

 which rises on either side of the middle line of the 

 back into a series of rounded lobes, each perforated 

 for the passage of an ambulacral tube and corre- 

 sponding therefore with an ambulacral foot. The 

 upper pair of vessels of the trivium send out series 

 of leaf-like sacs, richly loaded with pigment, which 

 fringe on either side the ambulatory disc, and ap- 

 pear to be chiefly concerned in the function of 

 respiration. 



" Sea-pens and Gorgonise have occurred frequently, 

 always remarkable for their brilliant phosphorescence. 

 Captain Maclear is giving special attention to this 

 beautiful phenomenon. A Mopsea, which shone 

 very brilliantly, gave a spectrum extending from 

 the green well on into the red, while Umbellularia 

 gave a very restricted spectrum sharply included 

 between the lines b and D of this wonderfully rare 

 sea-pen. We took with the trawl a very fine spe- 



* Ophiomustix?—'E.o.S.G. 



