December 30. 1897] 



NATURE 



201 



Thus it is gathered from Prof. Schaeberle's investiga- 

 tion that reflectors of large aperture must also be of 

 correspondingly great focal length if the definition of the 

 object to be observed is to be of the first order. 



As refractors of large aperture are seldom made of 

 such short focal lengths as are in question, it is unneces- 

 sary here to consider the effect of this source of bad 

 definition. 



Several interesting points with reference to the 

 capabilities of large instruments have been brought 

 prominently forward during the last few months, and may 

 be appropriately referred to in this place. Thus Prof. 

 Wadsworth {Astronomical Journal^ vol. xviii. No. 414) 

 has dealt with the efficiency of large refractors for visual 

 observations of planetary details. He finds that from 

 an optical point of view, it is a distinct advantage to in- 

 crease the apertures of telescopes intended for visual use 

 of planetary detail, such as fine linear markings, up to 

 such a point where the atmospheric aberration will amount 

 to about one-seventh or one-eighth period under the best 

 conditions of observation. If this point be exceeded, 

 then no advantage is obtained, the efficiency actually 

 falling off; the tendency is for the faint lines under 

 observation to be blotted out, instead of becoming more 

 distinct. Indeed so sure is he of this, that he says the 

 limit of efficient size is about reached between 30 and 35 

 inches, or the limit is very rapidly approached. 



Another point of great importance re large apertures 

 is that such telescopes cannot always be efficiently used 

 unless the night be very fine and the air still. The well- 

 known observer, Dawes, always used to judge the night 

 by the aperture that could be employed. Thus he spoke 

 of a one-inch night, two-inch night, up to an eight-inch 

 night, this being the greatest aperture he possessed. 

 As a matter of history, one may relate that a comparison 

 of the drawings of Mars made by Sir Norman Lockyer 

 with his six-inch refractor, and by Lord Rosse with his 

 big reflector, showed that although both series were made 

 at the same time, they displayed striking dissimilarities. 

 Dawes, who had also made some valuable drawings at 

 the same opposition, in discussing this question of dis- 

 similarity, concluded that Lockyer's drawings were the 

 more correct, since they were found to be exactly like 

 those he (Dawes) had made, especially with regard to 

 a certain marking which he had called the " double 

 tooth." 



Dawes, however, was no lover of large apertures, and 

 on the occasion just referred to he was heard to repeat 

 one of his favourite phrases, "What have the giants 

 done ? " 



Apart, then, from the quality of the instrument em- 

 ployed, definition depends on the state of the atmosphere 

 through which the light rays pass. On clear nights the 

 question of the movements of the air is of the highest 

 importance, and it is only on this movement that the 

 aperture for any particular night can be gauged. In 

 consequence of these air undulations, which vary in 

 different currents from half an inch to several feet in 

 length, the definition varies enormously. 



In the case of a small aperture, and supposing the 

 wave-length to be more than double the diameter of the 

 object-glass, the image of the object under observation 

 would only be bodily moved without confusion ; for a large 

 glass the image would be very considerably blurred. 



Dr. T. J. J. See has recently {Asir. Nach. No. 3455) 

 been making investigations on the sizes and movements 

 of these aerial movements, and his paper on this subject 

 indicates the importance of increasing our knowledge by 

 more systematic study. 



Thus it will be seen that in discussing the question of 

 how large telescopes may be made to do useful work, a 

 most important item to take into consideration is the 

 locality in which they will be used. If such a spot be 

 happily found, situated on a high plateau where the 



NO. 1470, VOL. 57] 



movement of the air is practically nil, then theoretically 

 there seems no reason why apertures should be limited 

 in size ; but as such a condition as this is rarely if ever 

 to be obtained, a limit is necessarily imposed on the 

 diameters of object-glasses. W. J. S. Lockyer. 



THE WO BURN ABBEY DEER. 



FROM the difficulty of obtaining an adequate series 

 of specimens, either living or dead, the deer are 

 one of the groups of large mammals with regard to 

 which our present state of knowledge is decidedly not 

 up to date, comparatively little advance having beer> 

 made since the appearance of the late Sir Victor 

 Brooke's well-known synopsis in the Proceedings of the 

 Zoological Society for 1878. Fortunately the noble owner 

 of Woburn Abbey, who takes a great interest in animals 

 of all kinds, is endeavouring to get together as complete a 

 collection as possible of these beautiful and interesting 

 ruminants, or rather of such kinds as experience shows to 

 be best suited to withstand the vicissitudes of the English 

 climate. With characteristic liberality the whole of the 

 magnificent collection now assembled is accessible to 

 zoologists interested in this group of animals, and by its 

 means considerable additions have already been made 

 to our knowledge thereof. From the extent of ground 

 much larger numbers of specimens of the same species 

 can be collected than is possible in the limited space 

 available in the Zoological Society's Gardens in Regent's 

 Park ; and the conditions existing in a large country 

 park are, of course, far more favourable to the well-being 

 and display of the animals than is possible in London. 



In the Regent's Park the larger kinds of deer, such 

 as the American wapiti, are generally, from necessity, 

 represented by only two or three individuals at a time, 

 but at Woburn these and other species are assembled in 

 herds of considerable size. And as deer are remarkable 

 for their seasonal variations in coat and colour, it is in 

 such manner only that a full grasp can be obtained of 

 these periodical changes. A further advantage is the 

 opportunity of seeing closely allied species or varieties 

 either in the same paddock or in near juxtaposition ; 

 while the facilities for studying the habits of the animals 

 are infinitely in advance of what is possible elsewhere. 



For a long period of years the domain at Woburn has 

 been a deer-park where large herds of red and fallow 

 deer wander at their own sweet will ; and the undulating 

 wooded ground alternating with level expanses of excel- 

 lent pasture, and the numerous lakes and ponds dotted 

 over the latter area afford an ideal situation for all 

 animals of this class. Such foreign species as adapt 

 themselves easily to these conditions, and do not make 

 themselves objectionable by developing habits of ferocity, 

 are allowed to run at large in the open park. Among 

 these are Pfere David's deer, of Northern China, the elk, 

 the Virginian deer, and the Japanese and Manchuriar> 

 sikas ; while muntjacs and roe run wild among the 

 coverts. Such an amount of liberty cannot, however, 

 be permitted to many of the species on account of their 

 dangerous propensities ; while it is found convenient or 

 necessary to afford more protection from the wind and 

 weather to yet other kinds. But even in the case of 

 species deprived of their full liberty, the amount of 

 space accorded them is ample, and quite different from 

 what is practicable in domains of smaller magnitude. 

 The American wapiti, for instance, live in a " paddock " 

 of about 150 acres, surrounded by an eight-feet iron 

 fence ; and in the same enclosure, as shown in our first 

 illustration, run the various races of sambar, as well as 

 some of the sikas, and various other small species. A 

 small herd of American bison are also among the 

 denizens of this enclosure. Hard by, in a paddock of 

 but little inferior dimensions, is a magnificent herd of 



