XII INTRODUCTION. 



questions, drew boundary-lines winch corresponded exactly with those which I had just drawn, 

 mentally, myself. Sometimes they differed from me ; but still the promptness with which they 

 designated such boundaries is proof that the transitions were perceptible to the eye. Generally, 

 much careful looking took place, and perhaps repeated attempts, before I ventured to draw my 

 lines. Often I was in doubt after all possible pains-taking, and the doubts are noted down. 

 As a general fact, late in the evening the Stronger Light would melt away gradually, or rather 

 would seem to be merged in the Diffuse Light, which alone would be left, the latter at first 

 witli a degree of brightness greater than it lately possessed ; and then the Diffuse would pass 

 away, in the increasing night. In the morning, the reverse was the case. It should be here 

 observed, also, that this gradation in the strength of the Zodiacal Light was not only lateral 

 from the centre outward, but also from the horizon upward to the terminating point. But the 

 transition, in this latter case, was by insensible degrees, except in the cases of a more intense 

 light near the horizon (to be noticed soon, and marked by zigzag lines on the chart); even the 

 Stronger Light, towards its apex, was so dimmed as to be distinguished with great difficulty, and 

 often I could make it out only by following up the boundary-lines from the lower portion. 

 Sometimes, towards the last of these observations, I declined drawing the apex at all on the 

 charts. (See Nos. 310, 314, &c.) 



I remember very well my feelings of surprise and wonder when the lateral changes in the 

 Zodiacal Light, as the night advanced, for the first time forced themselves on my attention. 

 Those changes, as may be seen from my charts, are of constant occurrence ; yet I do not find 

 them noticed in any writings on this Light, except an allusion by Cassini in one of his observa- 

 tions, in which, however, he tells us that, both then and afterwards, he could come to no certain 

 conclusions as to their existence.* 



These changes, running all through the observations, will be found to be of great consequence 

 when we come to draw conclusions from our data. They are greatest and most striking when the 

 ecliptic has declined considerably towards the horizon; and there is great uniformity in them, 

 but they are not without contradictions among themselves ; as, for instance, No. 177, t where the 

 lateral change ought apparently to be on the other or northern side ; but these incongruities are 

 rare, and are probably owing to extraneous causes. J I would not advise any one to draw con- 

 clusions from exceptions, in a matter where mistakes can be so easily made by the observer, but 

 only from the general facts of this book ; I have put down all, exceptions and incongruities as 

 well as others, not feeling authorized to omit any portion; for who can say, in a new science, 

 that what seem to be exceptions are not a part of the general rule ? 



Among the most important of these observations are those when the Zodiacal Light was seen 

 near and at midnight, simultaneously on both the western and eastern horizons a circum- 



- Jo cloutai si die n'avait pas uu peu de mouvement particular vcrs le septentrion ; car Ics deux plus luisantes il'. \rics 

 qu'clle frisait an commencement par son cote septentrional, furent ensuite comprises dans cette clartg ; ce qui a e"te" depuis 

 mnl'miie' par Ics observations drs jours suivaus. Mais je ne fus pas en etre cntierenient assure* ni alors, ni apres plusieurs 

 jours, parceque rextremite" de cette darte" c'tait de tous cotes trop douteuse, a'afflaiblissant peu-'a-peu ; de sorte qu'il e'tait 

 cxtivmement difficile de la determiner pre"cisement. " MCmoires de I' Academic Bm/ale Aea Sciences, torn. riii. 



j So also No. 101, where the 3 o'clock observation does not harmonize with the rest. 



{ Cassini remarks on the character of the Zodiacal Light as follows : " H ne faut ne'anmoins prt'tendre re"duire les appa- 

 ram-es ile rette lumicrr a un iV^U- aiissi cxacte que 1'anneau ile Katurne, parcequ'il s'cn faut beaucoup qu'clle soit si bien 

 termiui : e rt qu'cllu ait autant de consistence ; e'tant asscz evident, par les Jiil'rn -no-s uccidentalcs qu'elle fait paraitre d'un 

 jour a I'autiv, qu'rlle re'.'oit des variations reelles, outre celles qui viennent des causes externes, comiue des diversrs de'yres 

 de la darts' de 1'air et du concours de la lumicre desastres, et uieme de la disposition <1 > \vux de 1'observatcur." Mimoires 

 il r.[:<itliiiin /i'</"''', '""'. ''"'. fl'- ll>3. 161. 



