of Natural Philosophy. 141 



no distinct image of an object could be seen by light thus 

 reflected. 



Prop. 17. Exp. The effects of a single dense medium, 

 bounded by a convex surface, on parallel, diverging, and con- 

 verging rays, can never be illustrated by a convex lens, 

 which produces two successive refractions, — one by a con- 

 vex surface of the denser, and the other by a concave sur- 

 face of the rarer medium. The lens presents the combined 

 result of the former part of prop. 17, and the latter part of 

 prop. 18. In particular, a convex lens can never render 

 converging rays " less converging," as is asserted in the 

 fourth paragraph under the Exp. 



Precisely similar remarks might be repeated concerning 

 the introduction of the concave lens to illustrate the several 

 cases of prop. 18. — Both these experiments, if introduced 

 at all, should have been placed after prop. 18; and the man- 

 ner in which each illustrates 6o^A propositions should have 

 been pointed out.* 



Prop. 22. Cor. 2. The corollary is right ; but the inves- 

 tigation which is given of it, is incorrigibly wrong. By 

 comparison with the figure, it will be seen that it giv€S the 

 position of the principal focus of a glass sphere within the 

 sphere ; and that of a sphere of water, coincident with the 

 hinder surface. The proper mode of proceeding would be, 

 first to determine the focus of parallel rays entering a den- 

 ser medium by prop. 22; and then to find by prop. 23, the 

 focus of rays converging (to the point just found,) when 

 passing out of a denser medium into a rarer, through a con- 

 cave surface of the rarer. 



Prop. 26. " The image will not be distinct, unless the 

 plane surface on which it is received be placed at the dis- 

 tance of the principal focus of the lens." For " princi- 

 pal," read — " corresponding to the distance of the object." 



Prop. 35. " Though the distance of the object from the 

 lens be varied, the image may be preserved distinct without 

 varying the distance of the plane surface which receives it." 

 The distance of the plane surface from what ? The second 

 mode of doing it, pointed out in the demonstration, is in- 



* Both these propositions have materially suffered in point of clearness, 

 ffona employing as diagrams sections of lenses, instead of media iiidenrjitc 

 in the direction towards which the rays proceed after refraction,. — as we!! 

 ■li from (lie inaccurate manner in which some of the line? are drawn. 



