The Header in requested to make the following corrections : 



Page. Line. 

 Essay. Volume. 



3 69 3, for Dec. 3, read Dec. 13. 



11, Jbr upom reflrf upon. 



7 73 12, Jbr points read its points. 



8 74 3, 4, «?e/e quatain. 



24, aj'ter ellipses or liyperbolas at which the density is constant; add the axes 

 of these curves may be considered as natural axes of coordinates. 

 16 82 2, for cos. jy, Jy read cos. ^y. 3y. 



16, for -4- = — cos. gr, reat/ — ^ = — cos. px. 

 dz dz ^ 



20 86 12, add and each ellipsoid or hyperboloid thus moving will in general have two 



such enveloppes. 

 22 88 3, after cutting the final rays perpendicularly, add these spheres will also 



have another enveloppe, perpendicular to the incident rays. 



d''V d'^V 



26 92 23, for y, -j— ^ , read y 



dx.dz dx,dz ' 



30 96 8, for gives, read give. 



31 97 2, for -J5-, read^ . 



d^y d^ 



32 98 5 from bottom.yor aY, rearfrfy. 



33 29 formula (U), for --Jd^+Pd^) ^^^^^ ...-^d.+^jdz) 



'•' — {dy+ qdz) .,.—{dx +pds) 



35 101 22, for ^y't', read ^y's'. 

 37 103 11, for {r.s,t), read {r, s, t). 

 24, for dp', dp, read dp', d<f . 



39 105 13, insert commas after and .. 



in equation of projecting plane, for x! — g' read s! » j. 



40 106 9 from bottom, yor include, reac? include nearly. 



41 107 4, for {!>'), read(E') 

 43 109 17, for in, read infinite. 



last line, /or (VII) read (VI). 

 45 111 7, add combined with the formulae of [27.] 



10, for [27.] read [29.] 

 49 115 9 from bottom, for (C") read (G" ) 



VOL. XV. 



