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XXXV. Observations itfjon the different Hypotheses that have 

 been proposed respecting the Nature of Light. By William 

 Crane, Jiin. M.D. Boston, Lincolnshire*^. 



JDy light, we are enabled to distinguish the objects that sur- 

 round us; to view the beauties of creation, and to determine 

 the pleasing vicissitude of day and night. When mankind first 

 turned their attention to philosophical pursuits ; to account for, 

 and understand the various changes that are continually taking 

 place on the globe which they inhabit, and the heavens sur- 

 rounding it; one of their earliest inquiries was to attempt the 

 discovery of that agent which enabled them to perceive these 

 phaeaoinena; an agent which, after the lapse of centuries, and 

 the exertions of the greatest philosophers that have appeared, 

 still remains to be discovered. Empedocles and Plato rank 

 amongst the earliest philosophers that treat ujjon this subject. 

 They supposed vision to be occasioned by particles flying off 

 from the object seen, and meeting with others that proceeded 

 from the eye. This crude conjecture, unsupported by argument 

 or experiment, was soon rejected, and another advanced by Py- 

 thagoras, who maintained that particles were emitted from the 

 body that became luminous, and entered the pupi! of the eye j 

 a doctrine which still holds a place in the writings of many phi- 

 losophers of the present day, Aristotle appears to be one of the 

 first that considered light and heat not to be the same. Light he 

 imagined to be the act of a transparent substance considered as 

 such, and not arising from any matter proceeding from the lu- 

 minous body: — this idea of Aristotle's gave rise to the Cartesian 

 doctrine. In this hypothesis it was considered that light was 

 only a property of the luminous body, exciting a clear and vivid 

 sensation in the observer; or that an invisible fluid was every- 

 where present, and set in motion by an ignited body, or other 

 means, to make objects visible. Nearly similar to this is the 

 doctrine of Hiiygens, who conceived that light was a subtile 

 elastic fluid filling all space, and rendering l)odies visi!)le by the 

 undulations into which it is thro\\^^. According to him, when 

 the sun rises it agitates this fluid ; the undulations graduallv ex- 

 tend themselves, and at last striking against the eye cause us to 

 perceive the sun. As this hypothesis has occupied tTie atten- 

 tion of philosophers more than any other, except Newton's, we 

 shall state the objections to it, after having noticed what Dr. 

 Young has said on the same subject. Euler was a strenuous 

 advocate in favour of this opinion of Huygens ; but appears, to 

 have been unable to support it against the theory of Newton, 



* Communicated liy tho Author. 



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