its physical condition. If the spectrum be a continuous one, consisting of rays of every color or degree of refrangibility, then the source of light is either a solid or liquid incandescent body; if, on the contrary, the spectrum be composed of bright lines only, then it is certain that the light comes from luminous gas; finally, if the spectrum be continuous, but crossed by dark lines interrupting the colors, it is an indication that the source of light is a solid or liquid incandescent body, but that the light has passed through an atmosphere of vapors at a lower temperature, which by their selective absorptive power have abstracted those colored rays which they would have emitted had they been self-luminous.

When Huggins first directed his telescope in August, 1864, to one of these objects, a small but very bright nebula, he found, to his great surprise, that the spectrum, instead of being a continuous colored band, such as that given by a star, consisted only of three bright lines.

This one observation was sufficient to solve the long-vexed question, at least for this particular nebula, and to prove that it is not a cluster of individual, separable stars, but is actually a gaseous nebula, a body of luminous gas. In fact, such a spectrum could only be produced

by a substance in a state of gas; the light of this nebula, therefore, was emitted neither by solid nor liquid incandescent matter, nor by gases in a state of extreme density, as may be the case in the sun and stars, but by luminous gas in a highly-rarefied condition.

In order to discover the chemical nature of this gas, Huggins followed the usual methods of comparison, and tested the spectrum with