INTRODUCTION. XIH 



stance never observed before.* I had not expected it, and the manner in which it carne upon 

 me is recorded in Nos. 93 and 94, with the care, also, to have other eyes than my own brought 

 to bear on the subject, and also my carefulness in watching the western and eastern skies 

 through all the changes of the light, from early in the evening till dawn. It is probable that 

 tli is appearance can never be seen except when the ecliptic at midnight is at right angles, or 

 nearly so, to the spectator's horizon ; which can only be the case where his latitude is equal to 

 the sun's declination, but on the opposite side of the equator. I saw this again in the follow- 

 ing year (No. 266, &c.) ; and in both instances the ecliptic was not only vertical, or nearly so, 

 at midnight, but bore east and west from me; but the latter circumstance, I presume, had noth- 

 ing tn do with the results. I have been puzzled to know by what kind of lines to designate the 

 boundaries of this midnight Light; for it was very dim, quite as much so as the Diffuse Light; 

 yet when L came to bound it by lines of dashes, I found they produced confusion when the Dif- 

 fuse Light itself was marked down ; so I gave it a line of alternate dashes and dots, and thus 

 it is designated in the charts. 



Some time early in 1854, 1 saw in a newspaper a brief notice of the pulsations in the Zodiacal 

 Light seen at the Kew Observatory; but as the newspaper did not state where they were ob- 

 served, or the authority, and as I had now been observing for a year without having noticed 

 anything of the kind, I set it down as an ocular deception, and the thing passed entirely from 

 my mind. But in March of this year (see No. Ill), I was surprised, one evening, at seeing the 

 Zodiacal Light fade sensibly away, dimmed to almost nothing, and then gradually brighten 

 again. This was repeated several times ; but the effect, after all, was to leave me only in 

 amazement and doubt. Subsequent nights, however, gave abundant exhibitions of this kind, 

 of which, with the times and changes, I have made ample records with the particularity that 

 the case required. It was a great satisfaction, after my return home, to find that Baron Hurn- 

 boldt had observed the same thing while in southern latitudes, though he thought it more 

 probable that it was owing to "processes of condensation going on in the uppermost strata of 

 air, by means of which the transparency, or rather the reflection of light, may be modified in 

 some peculiar and unknown manner." My records, however, will show that there is a regu- 

 larity of appearance at the closing off of these pulsations, which proves that they do not belong 

 .to so uncertain a cause as atmospheric changes, but to the nebulous substance itself. They seem 

 to intimate a great internal commotion in the nebulous matter, for they were too rapid to be 

 occasioned by irregularities in its exterior surface. 



I noticed them again the following year, but must refer the reader to my records and charts. 

 The changes were a swelling out, laterally and upwards, of the Zodiacal Light, with an increase 

 of brightness in the Light itself; then, in a few minutes, a shrinking back of the boundaries, 

 and a dimming of the Light ; the latter to such a degree as to appear, at times, as if it was quite 

 dying away ; and so back and forth for about three-quarters of aji hour ; and then a change still 

 higher upwards, to more permanent bounds. 



A reference to the charts will show zigzag lines in some of them (see Nos. 288 and 323, and 

 almost passim) down near the horizon. These are the boundaries of a very effulgent light which 

 appeared at the times specified, and within these bounds. It has no other distinction than its 

 greater brightness, and the cause of it I cannot surmise. Cassini appears to have noticed the 



Unless, indeed, we cli-s this with what a German writer calls, the gegenschein ; for which sec notes to observation of 

 August 5th, 1S.">3. 



