840 



LECTURE XXXVIL 



ON PHYSICAL OPTICS. 



HAVING examined the general theory of optics, and the construction of 

 optical instruments, we are now to consider those properties and affections 

 of light, which rather belong to its natural history than to its mechanical 

 effects ; to trace its relations to the particular phenomena of nature ; to 

 investigate the manner in which it is connected with our sensations, and 

 to inquire on what intimate mode of action the various effects of light 

 depend. All these subjects may be properly comprehended under the 

 denomination of physical optics, but we shall find it convenient to reserve 

 each of the two last for a separate examination. The sources of light, the 

 velocity of its motion, its interception and extinction, its dispersion into 

 different colours ; the manner in which it is affected by the variable den- 

 sity of the atmosphere, the meteorological appearances in which it is 

 concerned, and the singular properties of particular substances with regard 

 to it, will be the first subjects of our investigation. 



The sources from which light is commonly derived, are either the sun or 

 stars, or such terrestrial bodies as are undergoing those changes which 

 con^itute combustion. The process of combustion implies a change in 

 which a considerable emission of light and heat is produced ; but it is not 

 capable of a very correct definition : in general it requires an absorption, 

 or at least a transfer, of a portion of oxygen : but there appear to be some 

 exceptions to the universality of this distinction ; and it has been observed 

 that both heat and light are often produced where no transfer of oxygen 

 takes place, and sometimes by the effect of a mixture which cannot be 

 called combustion. 



Light is also afforded, without any sensible heat, by a number of vege- 

 table and animal substances, which appear to be undergoing a slow decom- 

 position not wholly unlike combustion. Thus decayed wood, and animal 

 substances slightly salted, often afford spontaneously a faint light, without 

 any elevation of temperature ; and it is not improbable that the light of the 

 ignis fatuus may proceed from a vapour of a similar nature. 



The effects, which are commonly attributed to the motions of the elec- 

 trical fluid, are often attended by the production of light ; and violent or 

 rapid friction frequently seems to be the immediate cause of its appearance. 

 But it is difficult to ascertain whether friction may not be partly concerned 

 in the luminous phenomena attributed to electricity, or electricity in the 

 apparent effects of friction. Light is sometimes produced by friction with a 

 much lower degree of heat than is required for combustion, and even when 

 it is accompanied by combustion, the heat produced by the union of these 

 causes may be very moderate : thus it is usual in some coal mines, ,to 

 obtain a train of light by the continual collision of flint and steel, effected 

 by the machine called a fire wheel, in order to avoid setting fire to the 



