researches of three distinguished English men of scienceMr. Huggins, Dr. Miller, and Mr. Lockyer-to whom we are indebted for almost all our knowledge of celestial chemistry.

Although the moon and planets, shining by borrowed light, do not reveal to the spectroscope the nature of the material of which they are composed, like the sun and stars, yet something may be learned by an examination of the spectra of these bodies. You will remember that some of the dark lines in the solar spectrum are caused by absorption in our own atmosphere: now if an atmosphere of a similar kind exist round the moon or planets, the atmospheric absorption lines must appear more intense in the light reflected from these luminaries than they do in the light which passes through our air alone. With regard to the moon, the observations of Mr. Huggins and Dr. Miller have been negative. No signs of a lunar atmosphere presented themselves. A still more delicate means of ascertaining whether the moon possesses an atmosphere was employed by Mr. Huggins. On January 4th, 1865, he observed the spectrum of a star at the moment the dark edge of the moon passed over it. If an atmosphere existed in the moon, the observer would see the starlight by refraction after the occultation had occurred-just as the setting sun is visible to us after it has actually disappeared below the horizon. The variously coloured rays are, however, differently refrangible; and if any atmosphere existed round the moon, the red rays being least so would die out soonest, and the spectrum of the star would be seen progressively to diminish in intensity, beginning from the red end. Mr. Huggins observed nothing of this kind, all the rays of the stellar spectrum disappearing simultaneously: and

the conclusion must be drawn that the moon is devoid

In the spectrum of Jupiter lines are seen which indicate the existence of an absorptive atmosphere about this planet. These lines plainly 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 vapours similar to those which float about the earth. Another band has no counterpart amongst the lines of absorption of our atmosphere, and tells us of some gas or vapour which does not exist in the earth's atmosphere. From observations upon Saturn it appears probable that aqueous vapour exists in the atmosphere of this planet, as well as in that of Jupiter. In Venus no intensifying of the atmospheric lines could be observed; but some remarkable groups of lines, corresponding to those seen when the sun is low, were noticed on the more refrangible side of the line "D," in the Mars spectrum; and these indicate the existence of matter similar to that occurring in our own atmosphere. The red colour which distinguishes this planet appears not to be caused by absorption in its atmosphere, as the light reflected from its polar regions is free from the ruddy tint peculiar to the other portions of the planet. Padre Secchi and M. Janssen have likewise made similar observations, and they also conclude that in all probability the vapour of water exists in the planetary atmospheres.

I must now pass on to the subject proper of this day's discourse, which is to consider the properties of the light from the fixed stars. The more we learn about this subject, the more I think we must be surprised at the

accuracy of the observing powers of those philosophers who have given us this information. By means of this beautiful instrument (made by Mr. Browning, and a facsimile of the one used by Mr. Huggins, Fig. 78) we have been placed in possession of facts respecting the composition of the atmospheres, and the physical constitution of these stars, as accu

rate as the knowledge we possess concerning the composition of the solar atmosphere. It would be impossible for me to give you, even if time permitted, an accurate description of the method employed by Mr. Huggins. (See Appendix A.) Suffice it to say, that at the end of his telescope he has placed this spectroscope, containing two prisms (h h); and that, by very accurate adjustment, he is able to

bring the image of the star on the slit of his spectroscope (d). You may imagine how difficult these observations are, when you remember that the light of the star emanates from a point,-that is to say, the star has no sensible magnitude; that the image of the star has to be kept steady upon a slit only the so part of an inch in breadth; and, moreover, that the effect of the earth's motion has to be counteracted. When you add to this, that the amount of light which even the brightest stars give is excessively feeble, that this line of light must be still further weakened by being spread out by a cylindrical lens (a) into a band, and when you remember that in our climate on a few only of those nights in which the stars appear to the naked eye to shine brilliantly is the air steady enough to prevent the flickering and confusion of the spectra, fatal to these extremely delicate observations, I think you will easily understand how exceedingly difficult these researches must have been, and I am sure you will acknowledge the debt of gratitude which the world owes to those gentlemen who, by devoted labours, have brought the subject to this interesting issue.

In order to get a knowledge of the chemical composition of the stars, or to ascertain what chemical elements are present in them, it is necessary to use excessively delicate arrangements, by which not only the light from the star is allowed to pass through the prisms and to be received on the retina, but also that emitted by the various substances, the presence or absence of which in the stellar atmosphere it is desired to ascertain. These rays must pass together with the beam of starlight, or rather over or under the starlight, into the eyepiece, through the same prism, so that we may be able to