NOVEMBER 22, 1906| 
NATURE 81 
of known species in the fauna to 106. Contributions 
to the knowledge of the plankton are given by Dr. 
Geza Entz; he describes twenty-three species of 
Peridiniacez, and figures the seasonal variations of 
Ceratium hirundinella, which lives in the lake 
throughout the year, and is common from May to 
November. 
The last part of the whole work, the bibliography, 
has been compiled by Dr. Julius von Sziklay. It 
enumerates all the independent works, with sum- 
maries of their contents, and has special sections for 
maps and for contributions in journals and serials. 
The Hungarian Geographical Society is to be con- 
gratulated on this valuable contribution to natural 
science. The monograph on Lake Balaton will be 
of value to all students of the natural history and 
geography of Central Europe, and its summary of 
modern methods of research will be of use to students 
of lakes elsewhere. Moreover, the description of the 
country, revealing the quiet charm of its scenery and 
the many interests in the life of its people, should 
lead more visitors to this attractive district. 
A, 100-INCH REFLECTING TELESCOPE. 
Ne time passes and astronomical work advances 
there is a greater demand, year by year, for 
more powerful instruments of research. Fortunately, 
instrument makers have so far been able to fulfil the 
requirements for large refractors and reflectors, but 
a few years ago the time seemed to be reached when 
further progress appeared a long distance off. At the 
present day there are refractors in existence the ob- 
ject-glasses of which are as large as 30, 36, and 40 
inches in diameter, while the greatest glasS mirror 
that has been used measures 60 inches in diameter. 
In the case of the largest refractor, namely, that 
erected in the Yerkes Observatory in America, it 
seems possible that the size of this form of telescope 
has nearly reached its limit. The reasons for this 
are that, not only is it extremely difficult and costly 
to cast and figure lenses of such dimensions, which 
to give the best definition must be practically flaw- 
less, but the mounting has to be so immense and 
strong, and consequently very expensive in proportion. 
It must be remembered that in the refracting form 
of telescope the object-glass has to be placed at the 
upper end of a long tube, while the observer talxes 
his place at the lower end; these ends have to 
be very rigidly connected together, and the whole 
tube mounted so as to be capable of being moved in 
any direction. Thus in the case of the Yerkes tele- 
scope the tube had to be 62 feet long (weighing six 
tons), and the whole mass of metal that required 
moving every time the telescope was required in a 
different position was twenty tons. This will give 
some notion of the engineering difficulties that are 
involved in large refractors. In reflectors, on the 
other hand, the mirror is placed at the lower end of 
a comparatively light tube, and as close as possible 
to the mounting on which it is carried. In fact, in 
the case of the late Dr. Common’s 5-feet reflector a 
means was adopted of actually floating the mirror. 
Jn considering, therefore, the construction of tele- 
scopes much larger than those that already exist, 
attention is naturally being paid more to the reflecting 
type than to refractors. Further, it is not necessary 
that the glass casting for a mirror should be so 
perfect as that required for an object-glass, for in 
the former case only a perfect reflecting surface is 
required, while in the latter the light has to pass 
through the whole mass of glass. It is obvious, then, 
that much larger discs of glass can be made which 
may be suitable for reflectors but useless for refractors. 
NO. 1934, VOL. 75 | 
Aperture for aperture, a mirror costs about one-tenth 
the price of an object-glass, and this gives some idea 
of the extra work and risk involved in producing 
a good object-glass. 
The expense attached to the mounting of a reflector 
is also considerably jess than that of refractors when 
large instruments are in question. 
Now, not only is the reflector the less expensive 
of the two forms of instruments, but it has many 
distinct advantages optically. Thus chromatic aber- 
ration is a thing unknown in reflectors. Again, light 
being totally reflected from the silvered surface of a 
mirror is not lost like it is in refractors, where it 
always has to pass through the object-glass, and is 
consequently partially absorbed. 
Mirrors are, however, easily tarnished and affected 
by changes of temperature, but these disadvantages 
do not counterbalance the many points in their 
favour, to which reference has been made, when ex- 
ceedingly large instruments are under consideration. 
In the Proceedings of the American Philosophical 
Society (vol. xlv., No. 182, p. 44, 1906) Prof. E. C- 
Pickering communicated a paper entitled ‘‘ An Inter- 
national Southern Telescope,’’ and in it he strongly 
advocated the erection of a large telescope of the 
reflector type. His proposal was that the telescope 
should have a diameter of about 84 inches, and should 
be set up in some locality such as South America or 
South Africa, where the observing conditions are con- 
sidered very favourable. Towards the end of the 
paper Prof. Pickering referred to the important work 
that could be accomplished by means of such a large 
reflector, and mentioned that the name of a donor 
‘could in no way be better immortalised than by 
associating it with such a real advance in the greatest 
problem to the solution of which the mind of man 
has aspired—the study of the sidereal universe.” 
We learn now from the current number of the 
Astrophysical Journal (vol. xxiv., No. 3, October) 
that Mr. John D. Hooker, of Los Angeles, who on 
former occasions has rendered financial assistance to 
astronomy, has presented to the Carnegie Institution 
of Washington the sum of forty-five thousand 
dollars to purchase a glass disc too inches in 
diameter, 13 inches thick, and 50 feet focal length, 
and to meet other expenses incident to its construc— 
tion. These latter will include the erection of a 
building in which the mirror can be ground, figured, 
and tested; the construction of a large grinding 
machine, with crane for lifting the mirror (45 tons); 
the provision of a 54-inch glass dise to be made into 
a plane mirror for testing purposes, and other 
necessary items. 
The large mirror is intended for use at the Solar 
Observatory of the Carnegie Institution situated on 
Mount Wilson, in California, and under the director- 
ship of Prof. G. E. Hale. This observatory has 
already a 60-inch mirror in its optical shop, and at 
the present moment it is being tested. In the case 
of the new too-inch reflector, we are told the St. 
Gobain Company expresses its deliberate opinion that 
such a disc, 13 inches thick, can be produced, and 
that the Company will be able to carry out the order 
which has been given to it. 
The grinding and figuring will be entrusted to 
Prof. G. W. Ritchey, and no unsurmountable difficulty 
is anticipated by him in bringing such a mirror to a 
high order of perfection. The 60-inch mirror, now 
nearly completed, is the largest he has yet attempted, 
and this is now nearly ready for mounting. 
At present no financial provision has been made for 
the mounting and housing of this 1oo-inch reflector, 
but as the mirror will take, as we are told, about 
four years to complete, there is no immediate hurry- 
