INTRODUCTION. 



XLIII 



condensed part. 



The lines of dashes, drawn in the same general course 

 as (lie preceding, are the lines of a dimmer Light border- 

 ing the Stronger, as if from a more diffuse portion of 

 the nebulous matter lying beyond the central or more 

 It is called, in my notes, the Diffuse Light. 



On some of the charts are zigzag lines, showing the 

 extent of a very effulgent light at the base of the 

 Stronger Light. Its boundaries, at the upper part, were 

 not very decidedly marked, yet sufficiently so to enable 

 me to get them with a good degree of reliability. 



Still beyond the Diffuse Light, there was, on some oc- 

 casions, an appearance which seemed not to be exactly a 

 positive light, but rather as if the sky was paled, or was 

 a little less dark than beyond. The boundaries of this 

 are noted by dotted lines. I consider this, however, only as from the outmost and most attenu- 

 ated portion of the nebulous ring, now making itself visible, in consequence of the ring beino- 

 brought more laterally to the eye than before. Such dots are also sometimes used at the apex 

 of the lines for the Stronger Light, and mean, there, a light scarcely perceptible by the eye. 



Lines of dashes and dots combined, represent the 

 boundaries of th^Zodiacal Light as showing itself deep 

 in the night, from about 11 o'clock till 1. This Light 

 was in appearance like the Diffuse Light in its dimmer 

 aspects, but had lower and narrower boundaries ; and I have chosen a distinct mark for it, in 

 order to prevent the confusion which I found would arise on my charts, when the Diffuse Light 

 itself appeared. 



The full lines on the charts, with letters of the alphabet attached, are boundaries when the 

 Stronger Light was subject to pulsations, or risings and sinkings of the Light, as noted on the 

 opposite page. 



When lines run into each other, us in No. 90 and passim, the common line must be considered 

 as belonging to both observations, as far as the horizon will admit. 



Sometimes the charts are too small to admit the zenith points, or the sun. In that case, 

 their position is given in the annotations opposite. 



In making out the horizon lines, &c., for Cassini's observations, I allowed, for the annual 

 precession of the equinoxes, 50J", or '2 22' since 1G85. 



The reader will easily make out the N. and 8. directions in these plates. In the evening 

 observations, North is at his right hand; in the morning, at his left. 



The time used in these annotations is, uniformly, mean time. I had to take my reckoning 

 from the timepiece at the cabin door, which, especially in our rapid transitions E. or W., was 

 sometimes in error by a few minutes. But this was of little consequence. The relative periods 

 during the observations were of more importance, especially in noting the pulsations: and in 

 these I was very particular ; so that the relative periods may be fully depended upon. 



