i8S 



NA TURE 



\yan. 3, 1878 



It would be easy to extend our notice on the animal 

 forms alluded to, but our space forbids. It is curious 

 that no vegetable life seems to have been met with in 

 depths below 100 fathoms. " No plants live, so far as we 

 know, at great depths in the sea ; and it is in all proba- 

 bility essentially inconsistent with their nature and mode 

 of nutrition that they should do so." But parasitic alga 

 have been detected in some of the deep-sea corals, and 

 we are a little surprised to see the position of the diatoms 

 queried ; surely their plant affinities cannot now be dis- 

 cussed, and without these little plants we fancy some of 

 the plant- eating deep-sea forms of animal life would be 

 badly off. Holothuroids are especially fond of them. 



The following general conclusions are arrived at ; — 



" r. Animal life is present on the bottom of the ocean 

 at all depths. 



" 2. Animal life is not nearly so abundant at extreme 

 as it is at more moderate depths ; but, as well-developed 

 members of all the marine invertebrate classes occur at all 

 depths, this appears to depend more upon certain causes 

 affecting the composition of the bottom deposits, and of 

 the bottom water involving the supply of oxygen, and of 

 carbonate of lime, phosphate of lime, and other materials 

 necessary for their development, than upon any of the 

 conditions immediately connected with depth. 



*' 3. There is every reason to believe that the fauna of deep 

 water is confined p iocipally to two belts, one at and near 

 the surface, and the other on and near the bottom ; leaving 

 an intermediate zone in which the larger animal form*, 

 vertebrate and inveit brate, are nearly o entirely absent. 



" 4. Although all the principal marine invertebrate 

 groups are represented in the abyssal fauna, the relative 

 proportion in which they occur is peculiar. Thus Mol- 

 lusca in all their classes, Brachyourous Crustacea, and 

 Annelida, are on the whole scarce ; while Echinodermata 

 and Porifera greatly preponderate. 



" 5. Depths beyond 500 fathoms are inhabited through- 

 out the world by a fauna which presents generally the 

 same features throughout ; deep-sea genera have usually a 

 cosmopolitan extension, while species are either univer- 

 sally distributed, or, if they differ in remote localities, 

 they are markedly representative, that is to say, they bear 

 to one another a close genetic relation. 



" 6. The abyssal fauna is certainly more nearly related 

 tfean the fauna of shallower water to the faunae of the 

 tertiary and secondary periods, although this relation is 

 not so close as we were at first inclined to expect, and 

 only a comparatively small number of types supposed to 

 have become extinct have yet been discovered. 



" 7. The most characteristic abyssal forms, and those 

 which are most nearly related to extinct types, seem to 

 occur in greatest abundance and of largest size in the 

 southern ocean ; and the general character of the faunae 

 of the Atlantic and of the Pacific gives the impression 

 that the migration of species has taken place in a nor* 

 therly direction, that is to say, in a direction correspond- 

 ing with the movement of the cold under-current. 



"8. The general character of the abyssal fauna re- 

 sembles most that of the shallower water of high northern 

 and southern latitudes, no doubt because the conditions 

 of temperature, on which the distribution of animals 

 mainly depends, are nearly similar." 



These volumes form a distinct contribution to Science, 

 and will certainly be welcomed by the scientific worker ; 

 and their interest to the general reader, who can pass over 

 the few technical descriptions of the new forms, will be 

 scarcely at all less. 



THE MODERN TELESCOPE^ 

 III. 



WE know that both with object-glasses and reflectors a 

 certain amount of light is lost by imperfect reflection 

 in the one case, and by reflection from the surfaces and 



* Continued from p. 127, 



absorption in the other ; and in reflectors we have gene- 

 rally two reflections instead of onp. This loss is to the 

 distinct disadvantage of the reflector, and it has been 

 stated by authorities on the subject, that, light for light, 

 if we use a reflector, we must make the aperture twice 

 as large as that of a refractor in order to make up for the 

 loss of light due to reflection. But Dr. Robinson thinks 

 that this is an extreme estimate ; and with reference to 

 the four-foot reflector now in operation at Melbourne, 

 and of which mention has already been made, he considers 

 that a refractor of 3373 inches aperture would be probably 

 something like its equivalent if the glass were perfectly 

 transparent, which is not the case. 



On the assumption, therefore, that no light is lost in 

 transmission through the object-glass, Dr. Robinson esti- 

 mates that the apertures of a refractor and a reflector of 

 the Newtonian construction must bear the relation to each 

 other of i to i'42. In small refractors the light absorbed 

 by the glass is small, and therefore this ratio holds approxi- 

 mately good, but we see from the example just quoted 

 how more nearly equal the ratio becomes on an increase 

 of aperture, until at a certain limit the refractor, aperture 

 for aperture, is surpassed by its rival, supposing Dr. 

 Robinson's estimate to be correct. But with specula of 

 silvered glass the reflective power is much higher than 

 that of speculum metal ; the silvered glass being estimated 

 to reflect about 90 per cent.' of the incident light, while 

 speculum metal is estimated to reflect about 63 per cent. ; 

 but be these figures correct or not, the silvered surface 

 has undoubtedly the greater reflective power ; and, accord- 

 ing to Sir J. Herschel, a reflector of the Newtonian con- 

 struction utilises about seven-eighths of the light that a 

 refractor would do. 



In treating of the question of the future of the telescope, 

 we are liable to encroach on the domain of opinion, and 

 go beyond the facts vouched for by evidence, but there 

 are certain guiding principles which are well worthy of 

 consideration. These have lately been discussed by Mr. 

 Howard Grubb in a paper " On Great Telescopes of the 

 Future." We shall take up his poinds seriatim, premising 

 that in the two classes of telescopes, retractors and re- 

 flectors, each possesses some advantages over the other. 



We may conveniently consider first the advantages 

 which the refractor has over the reflector. 



First, there is less loss of light with the former than 

 with the latter, as a rule, hence for equal " space-pene- 

 trating power" the aperture of the reflector must be 

 greater. This condition gives us a greater column of air 

 and consequently greater atmospheric disturbance. 



" The refractor having a tube closed at both ends, and 

 the reflector being open at the upper end, the condition of 

 air-currents is quite different in the two cases, to the 

 disadvantage of the reflector, for in it the upper end 

 being open, there is nothing to prevent currents of hot 

 and cold air up and down the tube, and in and out of the 

 aperture, and for this reason great advantage has been 



' Sir John Herschel, in his work on the telescope, gives the following table 

 of reflective powers : — 



After transmission through one surface'of glass not in contact 



with any other surface ©'957 



After transmission through one common surface of two glasses 



cemented together _ \ 'oao 



After reflection on polished speculum metal at a perpendicular 



incidence ... ... ••• ••• ... ... 0633 



After reflection on polished speculum metal at 45° obliquity... 0690 

 After reflection on pure polished silver at a perpenaicular 



incidence 0905 



After reflection on pure po'ished silver at 45° obliquity _ ... o'pio 

 After reflection on glass (external) at a perpendicular inci- 

 dence ... ... ... ... ..• 0*043 



The effective light in reflectors (irrespective of the eyepieces) is as 

 follows : — 



Herschelian (Lord Rosse's speculum metal) A. o 632 



Newtonian (both mirrors ditto) ... ... ... ... B. 0436 



Do (small mirror or glass prism) C. 0632 



Gregorian or Cassegrain D. o 399 



1A. 0-905 

 B. o 824 

 C. 090s 

 D. o 819 



