Sept. 5, 1872] 



NA TURE 



371 



Fishes of Ncia Zealand. Catalo<^iie, with Diagnoses of 

 the Species. By F. W. Hu'tton, F.G.S., Assistant 

 Geologist. Notes on the Edible Fishes. By James 

 Hector, M.D., F.R.S., Director. With 12 plates. 

 (New Zealand, Wellington, 1S72.) 



The geologists of New Zealand are of opinion that, 

 in order to acquire a knowledge of the inhabitants of 

 their country in past epochs, it is necessary first to know 

 what its present inhabitants are. This will appear rather 

 strange to some of their European brethren, who do not 

 seem to consider that the subjects have anything to do 

 with one another, and who usually keep them as far apart 

 as possible. But there can be no doubt, we believe, that 

 our Antipodal friends are right, and that a knowledge of 

 the extinct fauna of any coimtry must be preceded by a 

 sturly of its existent fauna. 



We have lately given our readers an account of two 

 lately-published works on ihe Birds of New Zealand. We 

 have now the pleasure of introducing to their notice an 

 excellent resume of the present state of our knowledge of 

 the fishes of the same country. This has been prepared 

 by Captain Hutton, author of one of the previously 

 mentioned treatises, under the direction of the enlightened 

 Chief of the Geological Survey of New Zealand, who him- 

 self contributes additional information to the volume of 

 a highly important nature, in shape of Notes on the Edible 

 Fishes of the Colony. 



The arrangements and nomenclature adopted by Capt. 

 Hutton are that of Dr. Gumher's " Catalogue of Fishes," 

 the most recent and by far the best authority on the sub- 

 ject. A few new species are introduced, discovered since 

 the issue of Dr. Gunther's work, and some imperfectly 

 known fishes mentioned, which Dr. Giinther does not 

 appear to have noticed. Thus the total number of New 

 Zealand fishes included in Captain Hutton's work amounts 

 to 141. There can be no doubt, however, that this num- 

 ber will be considerably augmented when more attention 

 has been devoted to the subject. Indeed, we believe that 

 even withm the past few months Dr. Giinther has de- 

 scribed some rather remarkable additions to the Ichthyo- 

 logical Fauna of New Zealand, which are not included in 

 the present synopsis. 



The greater number of New Zealand fishes are marine 

 species. The freshwater fish fauna is poor in the ex- 

 treme, although it included two remarkable forms belong- 

 ing, one to the true Siilinonidir, and the other to a nearly 

 allied group, which are the " Smelt" and " Grayling" of 

 the native fishermen. The former fish. Dr. Hector tells 

 us, is "delicious food," but does not attain sufficient 

 dimensions to make it of great impoitance as an article of 

 diet. But the native " grayling," which probably reaches 

 6 lbs. or 8 lbs. in weight, ought to be valuable, and we can- 

 not quite understand why, with such a fish available, it 

 was thought necessary to spend large sums in the en- 

 deavour to introduce European Sal/nonidce into the fresh- 

 water of New Zealand. 



In concluding the brief notice of a most useful work, 

 we should add that the twelve plates which accompany it, 

 and which give the outline of about fifty of the most 

 characteristic species, even if not of great scientific value, 

 will, we are sure, be of great assistance to the unlearned 

 colonist in his attempt to make out the correct names of 

 the native fishes. 



LETTERS TO THE EDITOR 



The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken of anonymous 

 communications. ] 



Radial Polarisation of the Corona 

 Dl'RlNi; the recent meeting of the British Association at 

 Brighton, it was casually remarked by Sir William Thomson 



that during the eclipse of last December, it had been observed 

 that the radial polarisation of the corona was greater at a small 

 distance from the sun than close to it. This observation he 

 thought was a very important one. 



My observations on this subject were published in the Pliilo- 

 soph:cal Mn^iizinc for last Maich, but I think Sir William Thom- 

 son's opinion may be taken as sufficient reason for thinking that 

 they would not be uninteresting to the readers of Nature. 



I have twice accompanied Mr. Pogson, the Government As- 

 tronomer at Madras, as polariscopic observer, in his eclipse 

 expeditions ; first, in August 1S6S, and secondly, last December. 



In August 1868 I used simply a Savart's pokriscope, attached 

 to a telescope, employing a diaphragm to limit the field of view. 

 On this occasion I found that the corona was radially polarised. 

 I shall enter, however, more fully into a description of the 

 method adopted last December, as the results then obtained, 

 while confirming those obtained in 1868, are, I think, of much 

 greater importance. 



The Savart was so adjus'ed as to show a white central band 

 between two dark ones, when the bands corresponded in direc- 

 tion with the plane of polarisation. This polariscope was, last 

 December, converted into a polar imeter, by causing the light to 

 be examined to pass first through four plates of crown glass, 

 mounted in a frame, moveable on an axis at right angles to the 

 direction of the bands. This instrument was fixed in front of the 

 e)e-piece of a small equator ially-mounted telescope. 



When the glass plates were perpendicular to the optical axis of 

 the instrument, they had, of course, no effect whatever upcn the 

 polarisation of the light passing through them ; but if the frame 

 carrying the plates were turned on its axis, the light passing 

 through them would lie polarised in a plane parallel with the 

 axis, and hence at right angles to the direction of the bands, and 

 the amount of polarisation so caused maybe calculated for any 

 angle the glass plates may make with their normal or zero posi- 

 tion by means of formulcp in Prof. W. G. Adams's paper, pub- 

 lished in the Philosophical Magazine for April 1S71. 



It is evident that if we are examining common light by this 

 instrument, turning the glass plates will cause black centred 

 bands to appear ; that is, the central band will be dark, and 

 between two white ores, exactly the reverse of what takes p'ace 

 when light polarised in the plane of the bands is examined by 

 the polariscope. If then while examining light polarised in the 

 plane of the bands, we turn the glass plates, we prevent that 

 polarised light from passing to the polariscope, and the bands 

 gradually disappear as the ang'e of the plates increases, and they 

 completely disappear when the plates a)e at such an angle as 

 would produce on ordinary light the same amount of polarisation 

 as that existing in the hght under examination. 



By knowing the angle through which the plates have to be 

 turned in order to extinguish the bands, we are able to measure 

 the amount of polarisation in any polarised light ; the instrument 

 forming what mav be called a differential polarimeter. Further, 

 it is evident that if we place our bands at right angles to the plane 

 of polarisation, we shall get black centred bands, which would 

 only be increased in intensity by turning the glass plates ; hence 

 we can only measure the polarrsaiion of light with this irstru- 

 ment when the direction of the bands coincides wi'h the plane of 

 polarisation. 



My own observations of the eclipse of 1868 led me to expect 

 to find radial polarisation. 



Prof Pickering thought that the polarisation he observed in 

 1869 was caused by the reflection of sunlight from the unobserved 

 portions of the earth into the atmosphere, and thence back to the 

 eye. The plane of polarisation would, in that case, be vertical 

 near the sun. I took the portion of the limb at about 90° from 

 the vertex, towards the south (ihat is near the southern limb) for 

 my observations, and carefully placed my instrument so that the 

 bands were radial to the sun before totality. The bands were 

 therefore horizontal, or nearly so. This was then a crucial ex- 

 periment as to the accuracy of Prof Pickering's view on the one 

 hand, or of my own previous determination on the other. 



Directly totality comnrenced the bsnds appeared, ar d they 

 were white centred. The plane of polarisation was, therefore, 

 horizontal and not vertical ; it was radial, and could not be caused 

 in the way imagined by Prof. Pickering. 



I next turned the glass plates till the bands disappeared, and 

 by this means measured the amount of polarisation, which as I 

 have before explained, could not be done had the plane of polar- 

 isation been at right angles to the plane of the bands. 



On examining the light from a portion of the corona at about 

 ten minutes distance from the limb, I found the polarisation 



