162 TlMEHRI. 
natural colours of a landscape or a face have hitherto 
failed. There is, however, a legend that in the earlier 
days of the art, a wretched looking lad left at an optician's 
shop in Paris a photograph in natural colours and 
promised to return and show how it was done ; but he 
never was seen again and so the secret was never 
divulged. 
It was in 1701 that Newton discovered the chromatic 
prism, as it is termed, and beyond the fact that a ray of 
sunlight admitted into a dark room through a round hole, 
and traversing a three-sided glass, gave a rainbow 
coloured band of light, nothing was done or much thought 
about it for a hundred years when it was found that if for 
a round hole a narrow slit was substituted, the 
coloured image was crossed by innumerable black lines, 
and that when light from bodies other than the sun 
was employed, the number and position of these dark 
lines differed accordingly as this or that source of light was 
used. The flames from spirit of wine in which a chemical 
salt had been dissolved were then used, and it was ob- 
served that each salt, lime, soda, potash— whatever it might 
be gave its peculiar spectrum. It would take too long to 
dwell more on this discovery which affords so delicate a 
means of chemical analysis that it is said that the two- 
millionth part of a grain of sodium — a more than homce- 
pathicdose — is sufficient to produce its characteristic band. 
The composition of the planets or their atmospheres has 
been ascertained by thus analysing the light they shed. 
I hope, however, that we may induce our friend, Mr. 
FRANCIS, some evening to shew us all about this 
" spectrum analysis " as it is termed. 
Polarization is a term, and the polariscope an instru- 
