248 ON LIGHT. 



physical properties of light, and the theories which have 

 been advanced for their explanation. This need not 

 prevent us, however, from appealing to the effects pro- 

 duced by such instruments, especially such as are in 

 most common use, and as can hardly be other than 

 familiar to most of our readers, such as magnifying 

 glasses (or lenses), telescopes, &c. It requires no 

 knowledge of geometry, for instance, or any acquaint- 

 ance with its application to theoretical optics, to enable 

 any one to form a perfectly just conception of the mode 

 in which the eye enables him to see, when his attention 

 is called to a photographic picture, and he sees it im- 

 pressed on its ground by the rays of light collected and 

 brought to a focus by that assemblage cf convex and 

 concave lenses in a camera obscura which the photo- 

 grapher uses for the purpose. The dissection of an 

 eye shows it to he such an assemblage, and tlie picture 

 it produces may be actually seen at the back of the 

 eye of an animal recently killed, by removing the 

 opake leathery coat which envelopes it, and disclosing 

 the retina. How the nerves of that tissue, indeed, con- 

 vey to the mind the perception of colour and form, is, 

 and will probably ever remain, a mystery; but is no 

 more so in the case of vision than of any other of the 

 senses ; from which vision differs only in its transcendent 

 refinement and the elaborate structure of that most 

 wonderful of all optical instruments by which form., as 

 well as colour and brightness, is brought within its range. 

 The latter quahties are probably perceived by animals 

 unprovided with eyes, such as i\\Qproteus aJiguiiius^ which 



