NATURE 



[May 29, 1890 



Hexagonal prisms of the mineral are of frequent occurrence ; 

 good, clear, regular-shaped specimens, i cm. (or more) in dia- 

 meter, coming especially from Cumberland. The sides of these 

 prisms are, of course, parallel to the optic axis ; and hence a pair 

 of opposite sides, if smooth and well developed, serve for two 

 of the plane surfaces required for the Bertrand prism (see 

 Nature, May i?, p. 52) without any alteration. Moreover the 

 crystal has a very strongly marked cleavage along the planes of 

 the fundamental rhombohedron ; and since these planes make 

 angles of almost exactly 45° with the optic axis, a pair of them 

 will supply the two remaining surfaces of the prism. 



If, then, we select a good hexagonal prism of calcite, as 

 shown below — 



and carefully cleave it in two places, ab and CD, and allow a beam 

 of ordinary light (preferably from an opal lamp shade) to enter 

 the side of the prism near A, it will be affected in its passage 

 through the crystal in the manner explained in my former letter 

 (ante, p. 53), and the usual pair of ring-systems will be visible to 

 an eye receiving the light emergent near D. 



If there are any slight imperfections in the natural plane sur- 

 faces, we can easily improve upon Nature by cementing thin 

 plates of glass upon them with Canada balsam. 



Queen's College, Oxford. H. G. Madan. 



Red Spot on Jupiter. 



On May 22 last, at I5h., I observed Jupiter through a lo-incb 

 reflector, power 252, and saw the red spot between the east 

 limb and centre of the planet. It was rather faint, and not 

 nearly so conspicuous as some of the belts in its vicinity. Ac- 

 cording to careful estimation, the spot reached the central 

 meridian of Jupiter at I5h. 35m. This is I5'5 minutes before 

 the time given in Mr. Marth's valuable " Ephemerisfor Physical 

 Observations of Jupiter," published in the Monthly Notices for 

 March 1890, p. 344. The difference proves that the motion of 

 the spot continues to accelerate in a slight degree relatively to 

 the mean rate of rotation of gh. 55™- 40'63s., adopted by Mr. 

 Marth in his recent ephemerides. Observers, therefore, 

 who wish to see the spot at mid-passage across the disk of 

 Jupiter must turn their telescopes upon the planet at least a 

 quarter of an hour before the predicted times based on the daily 

 rate 870° -27. 



During my observation on May 22 I saw a very dark, if not 

 black, spot of circular form threading its way across the disk, 

 and projected upon the northern half of the great north equatorial 

 belt. I ascertained by reference afterwards to the Nautical 

 Almanac that this object was the fourth satellite in transit. 



Bristol, May 25. W. F. Denning. 



Testing for Colour-blindness. 

 May I ask, in connection with the lecture of Mr. Brudenell 

 Carter, why those interested in the testing of colour- vision do not 

 avail themselves of a scientific instrument like Lord Rayleigh's 

 colour-box, wherein a given yellow has to be matched by pro- 

 portions of red and green adjusted by turning a handle over a 

 dial graduated on the back ; instead of contenting themselves 

 with crude methods, such as selection of coloured wools, which 

 cannot give results definite enough to be of much interest, even 

 if they were quite efficient in detecting the grossly colour-blind? 



Oliver J. Lodge. 



In Mr. Brudenell Carter's interesting paper read at the Royal 

 Institution on Friday, May 9, a physiological explanation is 

 suggested of an easily-verified fact of colour-perception. 



An eye with the pupil dilated is proportionally more percep- 

 tive of red rays than one with the pupil contracted. If we stand 

 at right angles to a window, or other light, shading the further 

 eye with the hand, and look at a piece of white paper with the 

 two eyes alternately, we shall find that, to the eye in the light, 

 the tint of the paper seems distinctly colder than to the shaded 

 eye. 



Dr. Waelchli's observations of the retinal red zone in birds, 

 surrounding the central green region, makes the cause of this | 

 phenomenon comparatively clear ; while the phenomenon itself \ 

 tends to prove that the distribution of colour-zones in the human 

 eye resembles that of birds. E. H. 



May 16. 



I DO not know whether the following will be considered too 

 trivial to be admitted as an illustration of Dr. Brudenell Carter's 

 lecture. 



About 30 years ago, being then an assistant master at a 

 school, I one day asked an older colleague (since dead), " Who's 

 that boy in the red cap ? " Several were standing in a line, 

 most of them with black caps. ." Red cap?" said my friend ; 

 " I don't see any red cap " (it was scarlet flannel) ; "I can see 

 the red head" — meaning another boy with so-called "red" 

 hair. 



The same gentleman could see no difference in colour between 

 the flower and the leaf of the Pirns japonica ; but it will be 

 observed that he distinguished " red " hair without difficulty. 



Otham Parsonage, Maidstone. F. M, Millard. 



NO. 1074, VOL. 42] 



Coral Reefs, Fossil and Recent. 

 I PUBLISHED a review of the third edition of Darwin's 

 "Coral Reefs," with appendix by Prof Bonney, both in the 

 Biologisches Centralblatt and in the Natunvisscnschaflliche 

 Rundschau of last year. This will, I think, considerably 

 modify the meaning of Prof Bonney's statements, published in 

 Nature of the 15th inst. (p. 53), according to which I was 

 ignorant of this work of his when I wrote my letter on " Coral 

 Reefs, Fossil and Recent." R. von Lendenfeld. 



Swallows at Sea, 



The following notes from my journal as to meeting swallows 

 at sea during the autumn migration may be of use to anyone 

 interested in that subject. 



Onboard ss. Po7't FzV^^r from Plymouth to Sydney, 1889. — 

 " October 31, lat. 16° N., long. 19° W, A quantity of swallows 

 flying about the ship evidently tired and very tame, perching 

 freely within a few feet of anyone. Look thin. A solitary 

 swallow or two were seen yesterday and day before in lats. 20° 

 and 24° N. respectively. 9 p.m., passed Cape Verde at a 

 distance of forty miles. 



" November 1. — The swallows roosted last night on board, 

 and left during the forenoon, with the exception of one or two 

 who remained on board all day. Saw a curlew and a wagtail at 

 noon, lat. 12° N., long. 18° W. 



" November 2. — One swallow still on board, roosted last night 

 on the poop. Calm. Saw a wagtail. 



" November 'i. — Five swallows and two martins about the ship, 

 apparently in very fair condition, so tame that they would perch 

 on one's hand ; had three or four sitting on my hand at once 

 sometimes. Noon, lat. 5° N., long. 14° W., about 150 miles 

 off land. 



" November 4. — Swallows left, could not see in what direction 

 they went. Picked up south-east trade, lat. 2° N." 



Herbert E. Purey-Cust. 



H.M.S. Ei^eria, Auckland, April 6, 



The Corolla in Flower-Fertilization. 



1 have noticed a curious fact in reference to the blue gentian 

 of the garden here that will interest you. This flower (like the 

 daisy) closes at night and opens in the morning, and is exquisitely 

 sensitive to the time of sun rising and setting (it is a lasting, and 

 with its bronzed throat an exquisite flower). The fact observed is 

 this, that, when visited by the large handsome bee that fertilizes 

 it, the beautiful widespread pentamerous flower closes gently on 

 the bee, if the insect effectually enters and fertilizes it, on 

 its passage to the honey of the five cups at the base of the corolla ; 

 and after the insect's exit, does not again unfold, if the fertilization 

 is complete, but remains a folded flower— a protection (shorn of 

 its beauty) for the precious seed-vessel and its maturing contents 

 within. JohnHarker. 



Hazel Grove, near Carnforth, May 13. 



