192 Sir John Herschel and Modern Astronomy. — |April, 
Herschel found the stars visible in a reflecting telescope of eighteen 
inches aperture to amount to 5,331,572. He concludes, however, 
that the number really visible in the telescope is much greater than 
this, as in many parts of the Milky Way the stars appear so 
crowded as to defy counting. 
Not only were all the observations, during his sojourn at the 
Cape, made by Sir John Herschel himself, but all the reductions of 
every sort which are found in the published volume were executed 
by himself. While at the Cape, Sir John Herschel observed, with 
his usual care, all the phases of Halley’s comet, and noticed,— what 
had indeed been previously observed with regard to comets,—an 
enlargement of volume taking place simultaneously with the recess 
of the comet from the sun. Durimg the interval between the 
25th January, 1836, and the 1st of the following February, the 
volume of the comet was found by him to have increased im the 
proportion of 1 to 41:605. Sir John Herschel argues, that as the 
comet approaches the perihelion, the action of the solar heat will 
be constantly transforming the nebulous matter of which it is com- 
posed into the condition of a transparent invisible gas; and as this 
process necessarily commences at the exterior of the nebulosity, 
where the solar rays impinge, the immediate consequence will be a 
diminution of the volume of the comet. After the passage of the 
perihelion, the radiation of heat from the surface of the more con- 
densed portion of the comet will not be sufficiently compensated by 
the solar heat, and the diminution of temperature hence arising, 
will occasion a precipitation on the surface of the nebulous matter 
suspended in a gaseous state in the atmosphere of the comet. This 
precipitation of nebulous matter will continue to go on, under the 
influence of the cooling process occasioned by the increasing distance 
of the comet from the sun, and the manifest result will be the rapid 
enlargement of the visible dimensions of the comet. According to 
the laws of equilibrium the lighter particles of the precipitated 
vapour will arrange themselves so as to form the superior stratum 
of the enveloping nebulosity of the comet. It is evident also that 
as this bounding stratum continues to diminish in density, it will 
attain a higher and higher elevation, while at the same time its 
increased tenuity will cause it to assume a more and more filmy 
aspect. * 
It has been argued, from the slight retardation observed in 
Encke’s comet, and from other phenomena connected with comets, 
that evidence has been obtained of the existence of an all-pervading 
“ether.” While on the subject of comets, it will be advantageous 
—especially as showing the kind of reasoning which Sir John 
Hershel brings to bear on hypotheses of this class—to quote a 
* ‘Memoirs of the Astronomical Society, vol. vi., p. 99. 
