170 DISSERTATION SECOND. [fart i. 



SECTION V. 

 OPTICKS. 



1. Optical Knowledge of the Ancients. 



On account of the rectilineal propagation of light, the 

 phenomena of opticks are easily expressed in the form of 

 mathematical propositions, and seem, as it were, spontane- 

 ously to offer themselves to the study of geometers. Eu- 

 clid perceiving this affinity, began to apply the science 

 which he had already cultivated with so much success, to 

 explain the laws of vision, before a similar attempt had 

 been made with respect to any other branch of terrestrial 

 phj'sicks, and at least fifty years before the researches of 

 Archimedes had placed mechanicks among the number of 

 the mathematical sciences. 



In the treatise ascribed to Euclid, there are, however, 

 only two physical principles which have completely stood 

 the test of subsequent improvement. The first of these is 

 the proposition just referred to, that a point in any object 

 is seen in the direction of a straight line drawn from the eye 

 to that point ; and the second is, that when a point in an 

 object is seen by reflection from a polished surface, the 

 lines drawn from the eye and from the object to the point 

 whence the reflection is made, are equally inclined to the 

 reflecting surface. These propositions are assumed as 

 true ; they were, no doubt, known before the time of Eu- 

 clid, and it is supposed that the discovery of them was the 

 work of the Platonick school. The first of them is the 

 foundation of Opticks proper, or the theory of vision by 



