574 
NATURE 
[APRIL 28, 1923 

instead of the provisional estimate ; the change is by 
no means unimportant, the original estimate having 
been considerably in error. 
Capella would have been slightly beyond the theor- 
etical resolving power of the too-in. if used in the 
ordinary way, though an elongation might have heen 
detected. Resolving power was actually gained by 
blocking out the mischievous central portion of the 
aperture. But now came the final step—to produce 
fringes with a greater path-difference than any tele- 
scope yet constructed could provide. In 1920 a 20- 
foot interferometer, designed by Michelson and Pease, 
was constructed, in which the two interfering apertures 
could be separated to a distance of 20 ft. This was 
used in conjunction with the roo-in. mirror, which 
helped to bring the two beams together to produce 
their fringes. One might say that Michelson was now 
employing a 20-foot mirror; only, since he was only 
intending to use two small areas at its edges, he 
economically constructed those particular areas ‘and 
left the rest of the mirror to imagination. 
On July ro, 1920, the great 20-foot beam was placed 
across. the telescope. On December 13 success was 
attained, and the diameter of Betelgeuse was measured. 
Its interference fringes had totally disappeared when 
the mirrors were at to ft. separation, although the 
other stars showed them. The deduced diameter was 
0-045”—about the same size as a halfpenny fifty miles 
away. 
Michelson’s visits to Mount Wilson were limited to 
the summer months, and he was not present when this 
result was obtained. When he returned in 1921 he 
found his collaborators much occupied in trying to 
find some plan of obtaining definite measures of the 
visibility of the fringes instead of vague judgments. 
He suggested the plan of using two apertures, one fixed 
and the other variable ; a difference in the size of the 
apertures reduces the visibility to a definite extent, 
depending on the ratio of the two apertures. Finally, 

a comparison apparatus was constructed with one 
square aperture of 4 in. and the other a square 
variable from 4 in. to zero, in order to afford a 
definite scale of visibility. “ 
In the early trials it took days to find the fringes, 
but as gradual improvements were made a few hours’ 
work on the first night of a series of observations 
sufficed ; the subsequent settings being made in a few 
moments. 
I need only touch very briefly on later developments. 
Diameters of Antares, Aldebaran, Arcturus, and 
Pegasi have since been measured. But, of course, the 
discs of most stars are far below the limits for a 20-foot 
instrument. Prof. Hale is now constructing a 50-foot 
interferometer, and it is estimated that thirty or forty 
stars will be within its grasp. There is no need for a 
large mirror, and the use of the roo-in. in conjunction 
with the first interferometer was rather a luxury. The 
50-foot is of different design, and will not depend 
on any other telescope. All the diameters of stars 
measured up to the present confirm very closely the 
theoretical values that had been predicted for them. 
The enormous actual size of these stars—the earth’s 
orbit could be placed entirely inside Betelgeuse—is 
a picturesque feature of the results; but that was 
a confirmation of facts already established almost 
beyond doubt. 
It is not unlikely that interesting bypaths may be 
opened up. Considerable fluctuations in the diameter 
of Betelgeuse have been found, which may or may not 
be due to varying definition. The star is an irregular 
variable showing also changes of line-of-sight velocity, 
and the correlation of varying diameter with these 
other fluctuations would be of great physical interest 
if it turns out to be genuine. Michelson has pointed 
out that it is theoretically possible to determine by the 
interferometer the distribution of light over the dise— 
the law of darkening at the limb. That is a conceivable 
development for the future. 
Sunlight and Disease.! 
By Dr. C. W. SaLErsy. 
-\ | ex the beginning, God said, Let There Be Light.” 
In or before the eighth century ue! , Zarathustra, 
foremost among many sun-worshippers in many ages, 
taught the cult of the sun and the green leaf and 
thrift, in place of pillage and murder. In the beginning 
of medicine, Hippocrates, practising at Cos in the 
temples of Asculapius—son of Phcebus Apollo, god 
of the sun and medicine and music—practised the 
sun-cure. In the beginning of our era, Galen and 
Celsus used the sun. In the Dark Ages, by a pitiful 
misconception, the cult of the sun fell into desuetude 
as a species of pagan Nature-worship, and ill persons 
were treated alike in physical and in intellectual night. 
Tuberculosis and other ills were treated by the Sover- 
reign touch, reputed to cure the “ king’s evil.” 
In the second half of the nineteenth century, we find 
certain heralds of the dawn. In 1856, Florence Nightin- 
gale vigorously but vainly protested against the 
orientation of Netley Hospital, observing that no 
sunlight could ever enter its wards. In 1876, Sir 
} From a discourse at the Royal Institution on March 9, 
NO, 2791, VOL. IIT] 

Benjamin Ward Richardson praised sunlight in his 
“ Hygeia, The City of Health.” In 1877, Downes 
and Blunt showed that sunlight will kill anthrax 
bacilli. In many writings at this period, John Ruskin 
upheld sunlight and declaimed against the “ plague- 
cloud ” of smoke above our cities. In 1890, Dr. Theo- 
bald Adrian Palm (nat. 1848), who still practises 
medicine at Aylesford, in the Garden of England, 
showed by the ‘geographical method that lack of sun- 
light is the chief factor in the causation of rickets, 
and added an admirable series of recommendations 
accordingly.2, His paper was entirely ignored, and 
I found it in America, thanks to an American biblio- 
grapher. Robert Koch and others showed that sun- 
light kills tubercle bacilli. In 1893, Niels Finsen began 
to cure lupus, a form of cutaneous tuberculosis, by 
the local use of sunlight, and Sir James Crichton-— 
Browne made observations to the same effect in this — 
country. In 1900, on May 1, the London Hospital — 


2 “ The Geographical Distribution and A&tiology of Rickets," The Prac- 
titioner, October and November 1890. J 
