of Natural Philosophy. 143 



pair the respect of the student for its illustrious author. 

 The hypothesis of fits, however it may seem fitted to excite 

 ridicule as exhibited in this schohum, is now justly regarded 

 as one of the most striking displays which Newton ever 

 made of his transcendant genius. In the hands of Biot 

 and his companions in the career of discovery, it has ac- 

 quired an importance of wdiich Newton himself could have 

 had no adequate conception. — Whether the principles of 

 this now highly interesting and important department of Op- 

 tics can be reduced to the level of a system of elementary 

 instruction, is deserving of serious inquiry. A digest of the 

 phenomena and laws of polarization, involving no difficul- 

 ties which would render it inaccessible, or deprive it of its 

 interest with those who aim at nothing more than general 

 views of science, appears at least to be as yet a desidera- 

 tum. 



Prop. 58. " In all mirrors, plane or spherical, &;c." 

 This proposition, in regard to spherical mirrors, is true only 

 of those pencils of reflected light which are indefinitely 

 iiear the perpendicular. 



Prop. 69. In the demonstration it is stated that " by 

 prop. 31, the diameter of the image, when the object is 

 given, is inversely as the distance of the object." This is 

 not said, in prop. 31 ; nor is it true, except when the object 

 is very remote. The image formed by a lens is not in cir- 

 cumstances analogous to that produced on the retina of the 

 eye ; for the lens has no provision for preserving the image 

 distinct, for different distances of the object, without vary- 

 ing the distance of the plane surface which receives it. 



Prop. 73. " When equal objects in the same right line 

 are seen -obliquely, their apparent lengths are inversely as 

 the squares of their distances from the eye." The limita- 

 tion, " in the same right line," has been very properly in- 

 serted by the editor of the present edition ; but to render 

 it correct, it wants another limitation which the proposition 

 originally had as given by Rutherforth ; that is, " When 

 equal objects are seen t/'sry obliquely," &c. When the ob- 

 ject is of finite magnitude, the obliquity must be very great, 

 in order that the proposition may hold true, — unless indeed 

 the object itself be very small; in vchich case it holds true 

 for every degree of obliquity. But under this last modifi- 

 cation, it requires a different demonstration ; and is more 



