APRIL 18, 1884.] 
granules, merely as a minute fibrous network, or as 
films enveloping olivine cores, to that in which only 
minute particles of olivine survive as the nuclei of 
the granules, and to the final result of a true and 
complete serpentine.”’ 
Dr. Julien further claims, that the actinolites, am- 
phibolites, hornblende schists, and many of the talc 
schists, steatites, and serpentines of the Appalachi- 
an belt, are the equivalents of the dunite of North 
Carolina. 
The objections to some of Dr. Julien’s views have 
not been offered from any spirit of criticism of his 
truly excellent paper, but for the purpose of causing 
a more thorough study of the field-relations of this 
rock, and a presentation of the evidence that study 
affords. If the evidence, then, sustains Dr. Julien’s 
conclusions, his views will be accepted unhesitatingly. 
He has, indeed, given more evidence for his opinions 
than most writers on crystalline rocks are inclined to 
offer; for, as a rule, they appear to consider their mere 
dictum sufficient to prove the origin of any rock. It 
would seem that the time has come when statements 
regarding the origin of crystalline rocks cannot be 
accepted from any observer, unless these claims are 
accompanied by full and decisive proof of their cor- 
rectness. To bring about this healthy state in the 
study of the North-American rocks, the present 
writer has labored for years, and will continue to 
labor. M. E. WADSWORTH. 
ABOUT GREAT TELESCOPES. 
Dr. RALPH CoPELAND of Dun Echt, near Aber- 
deen, when returning to Scotland by way of this 
country a few months since, made a tour of several 
North-American observatories; and in a late number 
of Copernicus he contributes a paper on the Dudley 
observatory at Albany, the Litchfield observatory of 
Hamilton college at Clinton, the Warner observatory 
at Rochester, the Toronto observatory (Canada), the 
McGill college observatory (Montreal), the Harvard 
college observatory (Cambridge), the Winchester 
observatory of Yale college (New Haven), the two 
observatories at Princeton, and the U.S. naval ob- 
servatory (Washington). The noteworthy portions 
of the equipment of these establishments are briefly 
dealt with, and the work generally specified on which 
they areemployed. Dr. Copeland, having enjoyed the 
good fortune of seeing through a number of the finest 
telescopes in all parts of the world, places on record, 
at the conclusion of the paper, his general impres- 
sions of the actual state of telescope-construction on 
both sides of the Atlantic. 
First as regards their optical merits: it does not 
seem to him that any material difference as to the 
mere power of separating close double stars exists in 
the object-glasses made by the chief opticians in Eu- 
rope and America. On a night of good definition, 
any of their telescopes may be trusted to divide a 
fairly equal pair of stars at a distance indicated by 
Dawes’s table,! of which the following is a sufficient 
specimen : — 
1 Mem. roy. astr. soc., xxxv. 158. 
SCIENCE. 
487 
Aperture in inches. Least separable distance. 
1.0 4,56” 
2.0 2.28 
3.0 1.52 
4.0 1.14 
6.0 0.76 
8.0 0.57 
10.0 0.46 
12.0 0.380 
15.0 0.504 
20.0 0.228 
25.0 0.182 
26.0 0.175 
27.0 0.169 
30.0 0.152 
We thus see that in this respect our telescopes are 
practically perfect, and also that the atmosphere on 
the very best nights is sufficiently steady to permit 
their full power to be used. If, however, we test 
them on double stars, of which the components differ 
very much in brilliancy, then it is by no means so 
easy to come toa certain conclusion. There is the 
secondary spectrum to contend with, respecting the 
character of which it may be said that a certain de- 
gree of personal taste or fashion ‘exists. Some per- 
sons, notably opticians, seem to be little disturbed 
by a decidedly blue glare, while others prefer a wine- 
colored fringe. Perhaps, indeed probably, there is 
a physiological difference in the observers; for, if we 
suppose a person to be blind to the extreme blue and 
the violet rays only of the spectrum, to him an over- 
correeted object-glass would be perfect. With it he 
would be able to make out the closest companions 
of blue stars, or to see comparatively faint ones right 
up to the moon’s bright limb. ‘To such a person, 
however, an object-glass under-corrected to the same 
extent would appear to be a decidedly bad one. To 
Dr. Copeland, as well as to many of his colleagues, 
an average glass by Cooke or Grubb, and, to a less ex- 
tent, by Clark, appears over-corrected; while one by 
Schroder, and some of the Munich glasses, appear 
under-corrected. But here an important practical 
difference enters into consideration, one which has 
been particularly experimented on by Mr. Russell of 
Sydney; viz., that the correction of an object-glass 
may be lessened by separating the lenses: so that an 
over-corrected object-glass may be adjusted to any 
desired extent, while one that is under-corrected can 
only be used in the state in which it left the maker’s 
hands. As an example, it may be mentioned that 
the somewhat over-corrected object-glass of the 15- 
inch equatorial at Dun Echt has been materially im- 
proved by separating its lenses 0.2 of an inch, while a 
separation of 0.3 of an inch was found to throw out too 
much red about the primary image. This degree of 
improvement is best shown by the extremely linear 
character of the spectra of stars which it now gives. 
But in this connection it is only fair to mention, that, 
in making this object-glass, Mr. Grubb was limited 
to the relatively short ratio of 12 to 1 between 
the focal length and aperture. Opticians have not 
neglected to avail themselves of this property; and 
