1 6 
Dr. Young's Lecture on 
i( effluvia of a magnet can pass through a plate of glass, with-* 
“ out resistance, and yet turn a magnetic needle beyond the 
ec glass ?" (Optics, Qu. 18, 22.) 
HYPOTHESIS II. 
Undulations are excited in this Ether whenever a Body becomes 
luminous. 
Scholium. I use the word undulation, in preference to vibra- 
tion, because vibration is generally understood as implying a 
motion which is continued alternately backwards and forwards, 
by a combination of the momentum of the body with an acce- 
lerating force, and which is naturally more or less permanent ; 
but an undulation is supposed to consist in a vibratory motion, 
transmitted successively through different parts of a medium, 
without any tendency in each particle to continue its motion, 
except in consequence of the transmission of succeeding undu- 
lations, from a distinct vibrating body ; as, in the air, the vibra- 
tions of a chord produce the undulations constituting sound. 
Passages from Newton.. 
“ Were I to assume an hypothesis, it should be this, if pro- 
“ pounded more^generally, so as not to determine what light is, 
“ further than that it is something or other capable of exciting 
“ vibrations in the ether ; for thus it will become so general and 
“ comprehensive of other hypotheses, as to leave little room for 
“ new ones to be invented." (Birch. Yol. III. p. 24$. Dec. 1 675.) 
“ In the second place, it is to be supposed, that the ether is a 
« vibrating medium like air, only the vibrations far more swift 
* s and minute ; those of air, made by a man’s ordinary voice, 
,s succeeding one another at more than half a foot, or a foot 
