198 LECTURE ON THE RESULTS OF SPECTRUM ANALYSIS 



ties have been overcome, and satisfactory results obtained. We considered that 

 the trustworthiness of our results must rest chiefly upon direct and simultaneous 

 comparison of terrestrial spectra with those of celestial objects. For this purpose 

 we contrived the apparatus which is represented in the diagram. 



By this outer tube the instrument is adapted to the eye-end 'of the telescope, 

 and is carried round with it by the clock motion. Within this outer tube a 

 second tube slides, carrying a cylindrical lens. This lens is for the purpose of 

 elongating the round point-like image of the star into a short line of light, which 

 is made to fall exactly within the jaws of a nearly-closed slit. Behind the slit 

 an achromatic lens (and at the distance of its own focal length) causes the pen- 

 cils to emerge parallel. They then pass into two prisms of dense flint glass. 

 The spectrum which results from the decomposition of the light by the prisms 

 is viewed through a small achromatic telescope. This telescope is provided with 

 a micrometer screw, by which the lines of the spectrum may be measured. 



The light of the terrestrial substances which are to be compared with the 

 stellar spectra is admitted into the instrument in the following manner : 



Over one-half of the slit is fixed a small prism, which receives the light re- 

 flected into it by the movable mirror placed above the tube. The mirror faces 

 a clamp of ebonite, provided with forceps to contain fragments of the metals 

 employed. These metals are rendered luminous in the state of gas by the intense 

 heat of the sparks from a powerful induction coil. The light from the spark 

 reflected into the instrument by means of the mirror and the little prism passes 

 on to the prisms in company with that from the star. In the small telescope 

 the two spectra are viewed in juxtaposition, so that the coincidence and relative 

 positions of the bright lines in the spectrum of the spark with dark lines in the 

 spectrum of the star can be accurately determined. 



MOON AND PLANETS. 



I now pass to the results of our observations. 



I refer in a few words only to the moon and planets. These objects, unlike 

 the stars and nubulse, are not original sources of light. Since they shine by 

 reflecting the sun's light, their spectra resemble the solar spectrum, and the only 

 indications in their spectra which may become sources of knowledge to us are 

 confined to any modifications which the solar light may have suffered, either in 

 the atmosphere of the planets or by reflection at their surfaces. 



Moon. — On the moon the results of our observations have been negative. 

 The spectra of the various parts of the moon's surface, when examined under 

 different conditions of illumination, showed no indication of an atmosphere about 

 the moon. I also watched the spectrum of a star as the dark edge of the moon 

 advanced towards the star and then occulted it. No signs of a lunar atmosphere 

 presented themselves. 



Jupiter. — In the spectrum of Jupiter, lines are seen which indicate the exist- 

 ence of an absorpiive atmosphere about this planet. In this diagram these lines 

 are presented as they appeared when viewed simultaneously with the spectrum 

 of the sky, which, at the time of observation, reflected the light of the setting 

 sun. One strong band corresponds with some terrestrial atmospheric lines, and 

 probably indicates the presence of vapors similar to those which are about the 

 earth. Another band has no counterpart among the lines of absorption of our 

 atmosphere, and tells us of some gas or-vapor which does not exist in the earth's 

 atmosphere. 



Saturn. — The spectrum of Saturn is feeble, but lines similar to those which 

 distinguish the spectrum of Jupiter were detected. These lines are less strongly 

 marked in the ansae of the rings, and show that the absorptive power of the 

 atmosphere about the rings is less than that of the atmosphere which surrounds 

 the ball. A distinguished foreigner present at the meeting, Janssen, has quite 



