O. N. Rood on a Secondary Spectrum. 173 
that its dimensions vary with the amount of the disproportion- 
ality of the original constituents. Sir David Brewster seems to 
have made the most extended set of experiments on this sub- 
ject, and has given a list of substances, arranged in the order in 
which their spectra differ in this respect. 
is mode of experimenting was quite simple, consisting 
g 
ary spectrum can also sometimes be observed as a defect in 
telescopic or microscopic lenses; here also we have the green 
and purple fringes, but as in the other case, the phenomenon 1s 
quite inconspicuous, and is easily overlooked by those to whom 
it is for the first time presented. 
I have recently found it possible, by a proceeding of a differ- 
ent kind, to produce secondary spectra whose size is quite 
gigantic as compared with those just mentioned, and which dis- 
play the fixed lines with a distinctness that allows the study of 
their peculiar construction by ordinary spectroscopic methods. 
The essential condition for the production of a large secondary 
spectrum is a large difference in the spacing of the colors of the 
two primary constituents, and the largest secondary spectrum 
observed by Brewster was that furnished by a prism of sul- 
phuric acid in combination with one of oil of cassia, these sub- 
stances standing at the opposite extremes of his table. A vastly 
greater difference is, however, obtained, if we employ as one of 
the constituents the spectrum furnished by oil of cassia, bisul- 
phide of carbon, or even flint glass, the other being the normal 
spectrum obtained by the use of a diffraction grating. Here we 
approach very nearly, if we do not actually realize, the maxi- 
mum difference of spacing that is attainable in the present state 
of optical science, and hence give to the secondary spectrum Its 
maximum dimensions. 
Apparatus, &e.—A spectroscope provided with collimating 
and observin telescopes was employed ; it was provided wit 
a flat circular brass plate seven inches in diameter, which sup- 
d the prism and the grating; the vertical axis around 
Peed David Brewster’s Treatise on New Philosophical Instruments, p. 354: see 
his work on Optics. 
