NATURAL PHILOSOPHY. 107 



sons ; but they are all dull examples as compared to many chemical 

 precipitates. Precipitated chalk far outshines the natural varieties, 

 and fine qualities of magnesia carbonate surpass this. Microscopic ex- 

 amination indicates that this latter consists of particles, clear and 

 colorless, but very minute. White lead consists of particles equally 

 minute and also transparent, but of a yellow-brown color by trans- 

 mitted light ; consequently, when seen in bulk it appears of a less pure 

 white. But magnesia cannot be used as a pigment because it possesses 

 no body ; and the difference between the white lead and the magnesia 

 in this respect depends upon the different refractive powers of the in- 

 dividual particles which compose the separate powders. They are 

 both transparent in their individual particles, but the magnesia is more 

 so. Thy are both bodies possessed of considerable refractive power, 

 but the lead is more so. When air intervenes between their particles 

 the reflective power of both so much exceeds that of air, that they are 

 hi<rhlv reflecting and verv slio-htlv transmitting: ; but the less absorbing 



o v o ** O ( * <J f ~* i ~J 



power of the magnesia makes it the whitest, the more reflecting of the 

 two. But when oil intervenes, as would be the case if they were used 

 for pigments, the refractive power of the magnesia so nearly coincides 

 with that of the oil, that much transmission and little reflection is the 

 result, and this constitutes what painters call want of body. But the 

 lead so greatly exceeds the oil in refracting power that its reflective 

 property is not much interfered with, and even with its greater absorb- 

 ing power it reflects much and transmits little light ; and this is what 

 painters call great body. 



" The length of an undulation of violet light is seventeen millionths 

 of an inch; the red undulation is twenty-six millionths; undulations 

 longer or shorter than these not being visible. Again, the length of 

 the light wave varies in the medium. An undulation in air measuring 

 four will measure only two and a half when it enters glass, and will 

 again elongate to its former measure on its exit. When an undulation 

 passes from air into water, or into the humors of the eye, it likewise 

 becomes shortened. If we say that luminous undulations, which in air 

 measure twenty-two millionths of an inch, look yellow when they enter 

 the eye (that being the wave length belonging to what we call yellow 

 light), we must also remember that they measure one-third less in that 

 organ in consequence of its refracting power. We then come to the 

 singular conclusion that the blue sky is yellow, sunshine is red, and the 

 rosy tints of evening are not luminous at all till they enter the eye. If 

 the color depends upon the length of the light wave, and the length 

 of the wave depends upon the refracting power of the medium through 

 which it is passing, every beam of light changes color ; red it may be 

 on passing through the region of the stars, yellow or green it may be 

 when it enters our earth's atmosphere, blue or violet when it enters 

 water, non-luminous as it passes through glass. But if light, which we 

 perceive as violet while it exists in the aqueous humor of the eye, was 

 red originally, what color must that light be which we perceive is red ? 

 Its undulations in air must be too long to be luminous at all. This in- 

 troduces us to the solemn thought that all this vast universe is dark ! 

 Light only exists in the eye. It is only a sensation, a perception of 

 that which in nature exists as a force capable of producing a sensa- 

 tion." 



