SCIENTIFIC SUMMARY. 
309 
Henry Draper of the series of observations by which he has tested and con- 
firmed the evidence he had obtained in 1877 respecting the presence of 
oxygen in the sun. It will be remembered that the negatives he had 
obtained in 1877 were on a scale equal to about one-eighth that of Ang- 
strom’s normal spectrum, the enlargements being four times as great. He 
has now obtained negatives as large as the former enlargements, or on half 
the scale of Angstrom’s chart, so that the enlargements (fourfold as before) 
are on twice the scale of these reference charts. This is of itself a most 
important advance, indeed it may be doubted whether Professor Draper or 
anyone else will ever be able to pass much beyond this, seeing that the light 
of the electric spark in air which gives the comparison spectrum of oxygen, 
is scarcely equal to one standard candle. By comparison he finds that when 
the electric spark from his Gramme volatilizes iron, the light is sixty times 
stronger than the most vivid incandescence of air that he has been able to 
produce. Still further to improve the air spectrum, and thus to test the 
reality of the coincidences between the bright oxygen lines in that spectrum 
and bright parts of the spectrum of the sun, Professor Draper has devised 
what he calls the spark compresser. The condensed spark taken in the open 
air, or in a gas under atmospheric pressure, pursues, if unconfined, a zigzag 
course, and this is apt to produce a widening of the lines in the photographed 
spectrum. After many experiments, Professor Draper found that the spark 
might be compressed between plates of thick glass, or better still between 
two plates of soapstone. When the interval between the plates is directed 
towards the slit of the spectroscope the lateral flickering of the spark is pre- 
vented, while yet the spark is fully exposed to the slit without the inter- 
vention of glass or any substance on which the volatilized metal from the 
terminals could deposit. Very early in the research it became apparent that 
Pliicker’s tubes could not be employed with electrical currents of more than 
a certain intensity, partly on account of the deposit that took place in the 
capillary portion, and partly because the terminals became so hot as to melt 
and crack the glass. Moreover, it was desirable to use one terminal of iron 
so as to be sure that the spectrum of the gas was correctly adjusted to the 
solar spectrum, and this is impracticable with Pliicker’s tubes. An addi- 
tional advantage arises from the soapstone plates, viz., the temperature of 
the small volume of air between the terminals is materially increased, and 
increased brightness results. The comparison photographs thus enlarged 
and purified show as perfect agreement between the oxygen bright bands and 
the bright parts of the solar spectrum as did those obtained in 1877. The 
value of the evidence given by each coincidence is increased fourfold ; and 
it need hardly be said that that of the evidence from the complete series of 
coincidences is increased in much greater degree. In fact, as there are 
eighteen well-marked coincidences, it might theoretically be argued that the 
total value of the evidence is increased as (4) 18 to 1, or some 65,000 mil- 
lions of times. Although this number can hardly be taken as it stands, yet 
this may, at any rate be said, that if the evidence obtained in 1877 gave an 
even chance or even a small degree of probability in favour of the conclusion 
that there is oxygen in the sun, the evidence of the present series of photo- 
graphs makes this conclusion so exceedingly probable as to be practically 
certain. Professor Draper does not go quite so far as this, though he might 
