July 2, 1894.] 



KNOWLEDGE. 



l.ji) 



Dr. Hickson thicks that the darkness of the deep sea is 

 probably relieved by the brilliant phcsphoreEcence of many 

 of the deep sea tjsh. If, he says, we may be allowed to 

 compare the light of abysmal animals with that of surface 

 forms, it is possible that some regions of the deep sea 

 may be as brightly illuminated as a European street is at 

 night— an illumination with many bright centres and 

 many dark shadows, but quite sufKcitnt for a vertebrate 

 eye to distinguish readily, and at a considerable distance, 

 form and colour. 



To give an example of the extent to which the illumina- 

 tion due to phosphorescent organisms may reach, he 

 quotes Su- Wyville Thomson as stating " that on leaving 

 the Cape Verde Islands the sea was a perfect blaze of 

 phosphorescence. There was no moon, and although the 

 night was perfectly clear and the stars shone brightly, the 

 lustre of the heavens was fairly eclipsed by that of the sea. 

 It was easy to read the smallest print, sitting in the after 

 port in my cabin, the bows shed on either side rapidly 

 widening wedges of radiance, so vivid as to throw the sails 

 and rigging into distinct lights and shadows." All the 

 abysmal creatures have eyes, and the deep sea Crustacea 

 are uniformly coloured red, though the sun's light does 

 not give any perceptible illumination at a depth of even 

 two hundred fathoms. MM. Fol and Sarasiu, experiment- 

 ing with very sensitive bromo-gelatine plates, found that 

 there was no reaction after ten minutes' exposure at a 

 depth of four hundred metres on a sunny day in March. 



At a depth of two thousand five hundred fathoms the 

 pressure is, roughly speaking, two and a half tons per square 

 inch — that is to say, several times greater than the pres- 

 sure exerted by the steam upon the pistons of our most 

 powerful engines. Every animal at the bottom of the 

 Atlantic Ocean lives under a pressure about twenty-five 

 times greater than will drive a railway train. What this 

 enormous pressure is may be realized by the fact that 

 thick glass tubes, filled with air and sealed up at either end, 

 are crushed to powder by the water pressure before they 

 reach a depth of two thousand fathoms. 



Though l)r.Hickson's book contains an immense amount 

 of accurate information, it is comparatively light reading, 

 and may be confidently recommended. 



Practical Photi> ■ Mici-.n/mpln/. By Andrew Priugle, 

 F.R.:\I.S., &e. illiflfe and Son. London, 1894.)— Mr. 

 Pringle's book gives valuable practical instruction in 

 methods of research which are every day becoming more 

 and more important to biological students, as well as to 

 doctors and men of science generally. It does not profess 

 to give any history of micro-photography, or to treat 

 theoretically of the optical and photographic matters dealt 

 with, but the practical hints and suggestions to be found 

 on nearly every page will be welcomed by those who wish 

 to retain photographic records of the researches they are 

 engaged upon. 



ON THE CHANGES OF FORM IN COMETS' 

 TAILS. 



By A. C. Rany.vrd. 



VIDENCE is very rapidly accumulating which 

 tends to show that the matter of a comet's tail is 

 (at all events in the case of small comets) driven 

 away from the nucleus in a very irregular and 

 spasmodic manner, indicating that the action 

 going on in the nucleus as the comet approaches the sun 

 is by no means quiescent and regular, but rather cor- 

 responds in spasmodic irregularity with the outbursts 

 which accompany the rapid boiling of a liquid. There 

 seems to be a capricious irregularity about the explosive 



E 



phencmena we are familiar with in the action of terres- 

 trial volcanoes and also in the solar chromosphere ; 

 possibly Euch irregularities are always obEervable when a 

 rapid change of physical cccditicn is going en. 



Mr. H. C. Russell, the Director of the Sydrey Obseiva- 

 toiy, has kindly sent me seme photographs he has taken 

 of the Ccmet " Gale," with the accompanying series of 

 drawings showirg the remarkable manner in which the 

 tail of the comet appeared to Lim to vaiy in length ard 

 brightness from day to day. The presence or absence of 

 moonlight or hazy weather might have given lise to slight 

 diflerences in the apparent length of the tail, though none 

 actually existed, but it will not be contended that the 

 great changes shown in this series of drawings can be 

 accounted lor in any such simple manner. 



Rouijh Sketches of the Tail of I ' 

 April— Mai/, ls'94. 



iiiiet 



trille.' 



Date. 

 1R91. 



Diameter 

 of Coma. 



1° 

 I 



April 5 ' 

 „ 23 



,. 21 



2' 



6' 



10' 



3 IS' 



5 15' 



(i U' 



7 13' 



5 16' 



6 14' 



Mr. Russell carefully estimated the diameter of the head 

 of this comet on each occasion, and found the great and 

 irregular variations indicated in his diagrams. If we 

 could be sure that the nebulous and apparently spherical 

 envelope about the nucleus was gaseous, and that it was 

 held in equilibrium by the attraction of the nucleus, we 

 should have at our disposal a very satisfactory means of 

 estimating the mass of the comet, or rather of making a 

 minimum estimate of its mass ; for the envelope, if 

 apparently spherical, could not have approached the neutral 

 point between the sun and the comet, where the attraction 

 of the sun and the cometary nucleus coimterbalance one 

 another. A gaseous envelope about the nucleus held in 

 equilibrium by attraction, and extending to a height equal 

 to half the distance between the nucleus and the neutral 

 point would, if viewed from the direction in which we were 

 viewing Gale's comet, have appeared distinctly elliptical ; 



