INTRODUCTION. 



XXV 



boundary-lines toward, and at, the base, should be so affected, seems to be utterly inadmis- 

 sible. 



It is worthy of remark, also, how even and uniform, from apex to base, the change in the 

 boundary-lines is, as the hours change ; as if the substance giving the Zodiacal Light were not 

 only near, but also atone uniform distance from the spectator; the portions of it at the apex and 

 base of the Light all equally affected by his changes on the earth. 



We come now to apply to this case the two laws of light referred to in V; and I will give one 

 of them more in detail, leaving to the reader to apply them also to the diagram in V, accord- 

 ing as he may choose his positions there for the Zodiacal Light. 



A great number of experiments on the strength of light reflected at various angles have been 

 made in France, and are given by the experimenter, Mous. Bouguer, in his Traite d' Optique sur 

 la Gradation de la Lumiere. He says: "But what is more remarkable in the mathematical sur- 

 faces which we have just been considering, is the fact that the reflection is not equally bright 

 under all the angles of incidence.* In general, it is stronger under small angles, and more 

 feeble under larger ones. The difference is exceedingly great when the rays strike, with differ- 

 ent obliquities, the surface of very transparent bodies, but it is nearly as great in cases of cer- 

 tain opaque bodies; and I have never known it to fail altogether in any." 



Among the results of his experiments, he gives us the following tables, in which are shown 

 the number of reflected rays out of 1,000, falling, at various angles, on a smooth surface of water, 

 and on mirror-glass not quicksilvered. I have added a column converting his angles of inci- 

 dence into the angles between the incident and reflected rays. 



The want of entire certainty in applying such results to the present case of nebulous matter, 

 must be conceded; for we know not what nebulous matter is, and we cannot go beyond sur- 



= I'.y un^lr nf incidence, as he informs us, he means the angle between the incident ray and the HiirtVice of the reflecting 

 body "avcc la surface et nonptisavee saperpentliculnin ." 



