OG LIGHT. 



transmission, are well calculated to excite astonishment in those who do 

 not know by what means these results have been obtained. 



For a long time the question, What is the physical nature of light? 

 has occupied the attention of the most profoundly skilled and patiently 

 laborious investigators of the laws of creation, and with all the infor- 

 mation that has been elicited, no one perhaps would hazard the asser- 

 tion that it is even now decided. 



The first question which presents itself in reflecting upon the nature 

 of light is, in what manner is the perception of external objects produ- 

 ced ? It is well known that between the organs of vision and some 

 visible objects, .spaces intervene which, when compared with the small 

 terrestrial magnitudes to which we are accustomed, may be considered 

 as infinite, and yet, by some means, a connection is formed between the 

 organ and the object, so that a distinct impression or sight is produced. 



To account for this wonderful fact, two theories have been devised, 

 both of which have become celebrated from the distinguished names 

 which have been found in the lists of their advocates, and which are 

 known as the corpuscular and undulatory theories. 



The first theory supposes that light is material, that the luminous 

 body actually emits material particles which, far more minute than any 

 thing of which we can conceive, fly with a velocity almost infinite, and 

 impinging upon the delicate nervous net-work of the eye, produce the 

 impression which is called sight. 



The undulatory theory, on the contrary, denies the materiality of 

 light, and explains the effect by the supposition that an exceedingly sub- 

 tle, imponderable, etherial fluid pervades the universe, and that vibra- 

 tions are excited in this luminiferous ether, as it has been called, which 

 extend with immense rapidity in every direction, and produce the sen- 

 sation of sight. Amongst the supporters of this latter theory are found 

 the names of Huygens and most of the continental philosophers. 



That the transmission of light is not instantaneous, but that a definite 

 and measureable interval of time is required in passing through a given 

 portion of space, has not only been proved, but its exact velocity has 

 been measured. For this wonderful discovery the world is indebted to 

 Roemer, an eminent Danish Astronomer, to whom it was accidentally 

 presented wlulst engaged in an investigation of little importance when 

 compared vvith the treasure of truth to which it led. A series of observa- 

 tions on the eclipses of Jupiter's satellites furnished Roemer the neces- 

 sary data, from which to calculate the periods of their revolution and to 

 predict, with a prospect of certainty, the return of each successive disap- 

 pearance. To his surprise he found a discrepancy between the results 



