622 
NATURE 
[Oc¢. 28, 1886 
3000 kilometres from the photosphere without producing 
an amount of pressure at its base which certainly does not 
exist. Yet the spectroscope tells us that incandescent 
hydrogen is actually present at a couple of hundred times 
that height. Norcan the anomaly be reconciled by sup- 
posing, with our author, the solar atmosphere to be chiefly 
composed of a very much lighter gas (“1474”), merely, 
as it were, adulterated with hydrogen. Even if we were 
satisfied to ascribe to “ coronium” an almost impossible 
degree of elasticity, it would not avail to lift the mingled 
hydrogen one inch above its natural level. The law of 
diffusion does not abrogate the law of gravity. Each 
gaseous ingredient of a mixed atmosphere obeys its own 
law of equilibrium, as if it existed alone. It is true that 
the anticipated thinning out of oxygen at great heights 
in our own atmosphere has not been experimentally veri- 
fied ; but the incessant agitation of the air is believed to 
mask an effect which should otherwise be perceptible. 
The action of a repulsive force, such as is visibly exerted 
on comets, has been invoked as a means of escape from 
this difficulty. The supposition has much to recommend 
it,and would remove a good deal of perplexity ; it is, 
besides, countenanced by the authority of Dr Huggins. 
But the more tempting it appears, the more severely it 
should be tested previous to its admission, on other than 
a provisional footing, among the theories of science. 
The slightness of resistance to motion in the solar 
neighbourhood is one among many indications of the 
extreme tenuity of matter there. Comets well-nigh graze 
the sun’s surface without experiencing perceptible retard- 
ation; and millions of cubic miles of hydrogen sweep 
onwards or upwards at rates up to 250 miles a second, 
almost as if 77 vacwo. Since both the moving substance 
and the medium are incandescent, the varying viscosity | 
of gases at high temperatures would claim attentive con- 
sideration in the matter, were it not that reliable data are 
unfortunately deficient. 
The rotational peculiarity of the sun may, however, be 
said to dominate the problem of its constitution. Three 
classes of explanation are possible, and have found various 
degrees of favour. 
matter upon the photosphere, by the ascent of matter 
from beneath it, or by surface-currents. The last was the 
theory of Zéllner, and has been inherited from him by M. 
Schulz, but may be dismissed without hesitation as con- 
tradictory of known facts. M. Faye’s hypothesis of ver- 
tical currents bringing up with them a smaller linear 
velocity is more plausible, but needs peremptory treat- 
ment to fit it to the required shape. It is remarkable 
that M. Belopolsky has lately deduced theoretically, on | 
some not improbable assumptions, Sp6rer’s empirical 
formula of the diminishing rate of the sun’s rotation north 
and south from his equator (see NATURE, vol. xxxiv. p. 54). 
The coincidence is striking ; but it must not mislead us. 
It is not enough that a cause be true; it must also be 
sufficient. Is there any likelihood of its being such in 
this case? We apprehend that the effects, even sup- 
posing them realised to the full, would be microscopic 
compared with those actually observed. If we take the 
mean density of the sun at double its superficial density 
(an outside admission), the maximum of gravity will occur 
below the surface, at a depth of one-sixth of the radius, and 
there the ¢heoredica/ rotation-period comes out, by a rough 
calculation, about twenty-two days. But this period is 
not in reality complied with. The tremendous hindering 
power of friction intervenes. It occurs on paper only, 
and belongs even there but to a single stratum. The 
effects in superficial acceleration must be quite incon- 
siderable. 
Acceleration from below failing, we turn to acceleration 
from above. And it has to be borne in mind that the 
mode of the sun’s rotation is inferred from the movements 
of spots, and from them alone. But if spots be due, as 
generally supposed, to vaporous down-rushes, they must 
It may be produced by the fall of | 
share in the augmented velocity brought by the materials 
forming them from regions of wider circumference ; and 
this theoretical necessity is confirmed by the characteristic 
plunge forward attending the sudden development of these 
objects. If we assume further that the height of fall, con- 
sequently the added linear velocity, diminishes pro- 
gressively with distance from the equator, the phenomena 
of spot-transport in longitude are satisfactorily accounted 
for. Just such a graduated elevation of the sources of 
spot-supply forms an integral part of Mr. Lockyer’s 
“ meteoric-ring theory” of sunspots, communicated to the 
Royal Society in May last (NATURE, vol. xxxiv. p, 251). 
The line thus struck out, however it may be modified by 
future experience, seems to lead, more naturally and easily 
than any other yet tried, to the solution of the problem 
of the sun’s apparent rotation. A. M. CLERKE 
THE NEW OPTICAL GLASS 
VERYTHING that falls from the pen of Prof. Abbé 
of Jenarelating to optical matters commands respect. 
His announcement therefore of the discovery of new 
kinds of glass specially adapted for the manufacture of 
lenses has been received with peculiar interest; and 
although details of information on various points are still 
wanting, enough has been published respecting the new 
optical glass to be worthy of more than passing notice. 
All who have had anything to do with optical instru- 
ment-making know only too well the existence of the 
defect termed “irrationality of dispersion.” When Hall 
and Dollond had independently shown that the chromatic 
dispersion of a crown glass lens might be corrected by 
combining it with a second lens of flint glass, a new 
impetus was given to optical research. The so-called 
“achromatic” lens in the hands of successive generations 
of opticians constituted the basis both of the modern 
microscop2 and of the modern telescope. But as greater 
and greater perfection in the construction of the “ achro- 
matic” lens was attained, it became apparent that perfect 
achromaticity was very far from being realised; for 
though two lenses might be found which should perfectly 
bring together two widely differing rays, such, for ex- 
ample, as the red of the line C of the spectrum and the 
blue of line G of the spectrum, it by no means followed 
that this pair of lenses would bring together to the same 
focus all other rays. On the contrary, owing to the 
“irrationality” of dispersion a “secondary spectrum” 
would always remain uncorrected 
The relation between the chemical constitution of a 
medium and its action on waves of light of different 
periods is one of those matters about which we are still 
profoundly ignorant. We know that a prism of glass 
does not spread out the waves in proportion to their 
wave-length, or to the frequency of their periods. A 
simple case of irrationality is afforded in the fact that a 
green ray which when viewed through one prism may lie 
exactly half-way between C and G in the spectrum will 
not lie exactly half-way when viewed through a prism of 
a different kind of glass. All that a combination of two 
lenses can do is to achromatise for two rays of the spec- 
trum: it may very nearly achromatise for the neigh- 
bouring rays, but strictly speaking it only achromatises 
for two. For ordinary optical purposes we seek to 
achromatise for the red and the blue, so reconciling the 
end regions of the visible spectrum. For photographic 
purposes we achromatise for green and violet (or even 
ultra-violet) rays, reconciling the end regions of the 
photographically active spectrum. 
To Dr. Blair, whose observations were published in the 
Transactions of the Royal Society of Edinburgh for 1791, 
we owe the suggestion to achromatise for three rays by 
using compound lenses of three different media. Blair, 
indeed made a most extensive examination into the dis- 
persive powers of various media, and in particular of 
