January 1, 1890. 



KNOWLEDGE. 



51 



resort to the shore to deposit their eggs, the limbs have 

 all the toes enclosed in a common scaly integument, and 

 these form paddles admirably adapted for swimming 

 organs. While, however, the edible Green-tm'tle is of strictly 

 lierbivorous habits, the fierce Hawksbill is purely carnivo- 

 rous. Another very remarkable group of marine Chelonians 

 is now represented only by the so-called Leathery-turtle, 

 which differs fi'om all other living forms in that the shell 

 is represented only by a carapace composed of a number of 

 small bones closely joined together, and forming a mosaic- 

 like structure which is totally unconnected with the ribs. 

 The living Leathery-turtle attains a length of about five 

 feet ; Init some allied fossil forms are estimated to have 

 been as much as ten feet in length, and were thus veritable 

 aquatic giants. The largest existing representatives of the 

 true tortoises are, or were recently, found in certain islands 

 of the Indian Ocean, and also m the well-known Galapagos 

 (or Tortoise) Islands lying ofl:' the coast of South America. 

 A magnificent series of these giant land-tortoises is now 

 exhibited in the Natural History Museum at South Ken- 

 sington. Many of these huge creatures, which have in 

 some cases been completely exterminated by sailors, could 

 readily walk off with a man seated on-their back ; and in the 

 larger forms the length of the shell in a straight line is 

 upwards of four feet. These dimensions were, however, 

 vastly exceeded by certain fossil species found both in 

 Northern India, the south of France, and elsewhere ; the 

 shells of some of these monsters attaining a length of six 

 feet. The late Dr. Falconer, who was disposed to con- 

 sider that the size was considerably greater than this, has 

 indeed suggested that some of these huge tortoises may 

 have lived on in India into the human period, and thus 

 have given rise to the old Sansci-it legend that the earth 

 was supported by a gigantic elephant standing upon the 

 back of a stOl more gigantic tortoise ; the legend being 

 silent as to what constituted the support for the tortoise. 



In conclusion, we may say a few words as to the classifi- 

 cation of Chelonians, leaving, however, out of considera- 

 tion the leathery-turtle and its allies, in regard to the serial 

 position of which there has been been a considerable 

 amount of discussion. Ordinary Chelonians are readily 

 divided into two great groups, according to the manner in 

 which the head is retracted within the shell. Thus, in 

 the land-tortoises (Fig. 1) and their allies, the head is 

 drawn directly within the margin of the shell by the 

 bending of the neck in an S-like manner in a vertical 

 plane. The plastron of this group is generally charac- 

 terised by the absence of the unpaired mtergular shield 

 (Fig. :•!), so that the two gular shields meet in the middle 

 line. This group, as we have already mentioned, includes 

 all the tortoises of the Northern Hemisphere, with the 

 exception of the soft tortoises, and also the marine-turtles ; 

 it is, however, by no means confined to this hemisphere, 

 although it is totally wanting in Australia. The name 

 Cryptodirans, or hidden-neck tortoises, is applied to the 

 members of this group. In the second great group, on the 

 other hand, the neck is bent sideways, so that the head, 

 when retracted, lies on one side of the front aperture of the 

 shell near to one of the legs ; and the presence of an inter- 

 gular shield in the plastron (Fig. 41 is absolutely charac- 

 teristic. The members of this group are termed Plurodi- 

 rans, or side-necked tortoises, and, as we have said, are 

 now exclusively confined to the Southern Hemisphere, and 

 are the only C'helonians met with in Australia. In former 

 epochs this group was, however, nmch more widely dis- 

 tributed, from which we may probably conclude that the 

 Pleurodirans are an older and less specialised type than the 

 Cryptodirans. A very remarkable gigantic fossil tortoise 

 from Queensland and Lord Howe Island, which may pro- 



bably be regarded as a member of the Pleurodiran section, 

 is remarkable for having large horn-like prominences on 

 the skull, and also for the bony rings with which the tail 

 was protected, somewhat after the fashion of the Glypto- 

 dont Mammals (•«'(■ Knowledge for December). Finally, 

 the soft tortoises form a third group, allied to the Crypto- 

 dirans in the mode of retraction of the head, but distin- 

 guished by certain peculiarities in the structure of the 

 skttll and shell. From the presence of only three claws 

 on the feet of the tj'pical forms, this group is technically 

 termed the Trionychoidea ; and with tlais group we bring 

 our brief remarks on tortoises to a close. 



HARVARD COLLEGE OBSERVATORY. 



By A. C. Eanyaed. 



THE accompanying plate was made ft-om a photograph 

 sent me by Prof. W. H. Pickering. It represents 

 the rear of the celebrated Observatory of Harvard 

 College, w-hich is situated on a lull at Cambridge, 

 a suburb of Boston, Massachusetts. The photo- 

 graj^h was taken by Prof. W. H. Pickering without a lens. 

 The image which fell upon the sensitive plate was formed 

 by light from a jjin-hole in the front of the camera ; but 

 though the sun was nearly setting when the photograph 

 was taken, the exposure was only six minutes. The photo- 

 graph was taken from the roof of a cottage to the north- 

 west of the Observatory, and it will be noticed that the 

 slates of the roof, and the wires, and other objects in the 

 immediate foreground (with the exception of the fir trees, 

 which probably moved with the wiad during the exposure), 

 are equally as sharp as objects at a distance. There being 

 no lens, all objects are equally in focus and there is no 

 distortion, or rather there is the same distortion as there 

 should theoretically be in a hand-dra^Ti picture ; all objects 

 appear as they would be seen projected on the tangent 

 plane to a sphere by an eye situated at the centre of the 

 sphere. 



A photograph of any size may be taken with such an 

 arrangement, but the farther the plate is from the pin- 

 hole the larger will be the image of the object photo- 

 graphed, and the longer will be the time of exposure. 

 Lord Kayleigh has shown that photographs may be 

 obtained w-itlx holes considerably larger than pinholes, 

 provided the plate is removed to a distance of some feet 

 — the condition of sharp definition being that waves from 

 diS'erent parts of the hole shall not interfere. No per- 

 ceptible diminution in the sharpness of the image is 

 apparent when the distance of the plate from different 

 parts of the hole is less than a quarter of a wave length. 

 The small observatories in the foreground contain various 

 photographic instruments ; that to the right contains an 

 11 -inch photographic refractor, and the central dome 

 covers Dr. Henry Draper's 28-inch silver or glass 

 reflector ; the other domes and huts cover the instru- 

 ments with which Professor C. E. Pickering's photometric 

 survey of the heavens has been made, as well as the 

 instruments with which the series of photographs of 

 stellar spectra have been taken. 



The large dome at the centre of the main building 

 covers the 15-inch refi'actor formerly used by Bond, which 

 was in its day the largest instrument in America, and 

 shared with the old 15-mch refractor of the St. Peters- 

 burg Observatory the honour of being the largest 

 achromatic in existence. It was brought to America in 

 1847. and with it Bond discovered Hyperion, the 8th 

 satellite of Saturn, in September 1848 ; a few days after- 

 wards, Hyperion was independently discovered by Lassell, 



