192 
Proceedings of the Eoyal Society 
equally surprising to either Sir William Herschel, or Argelander or 
Struve up to within a very few years ago. 
The reason of this sudden development of astronomical photo- 
graphy is twofold. First, a remarkable improvement, amounting 
even to a bursting of its bonds of the previous quarter of a century, 
which has taken place in ordinary photography ; and second , the 
accompanying circumstance, that those new methods have been 
found most peculiarly suited to the special requirements of the 
Astronomer. 
The new Photography of the portrait gallery consists in the re- 
placement of inflammable collodions, the silver hath, with all its 
sicknesses, wet films, soft, slippery, contractile and perishable, 
together with sensitizations which each worker had to prepare pain- 
fully to his own hand, each time of using ; — their entire replacement 
by dry plates, prepared by the thousand at large manufactories, ready 
to any one’s hand, and with their bromo-iodised hard gelatine films, 
far more sensitive than the best wet collodion of the old silver hath 
preparation. 
Now, this enormous improvement has suited the astronomers in 
several ways. They were never very fond of dealing with alcohol 
and ether at night, or manipulating all the other dangerous, or deli- 
cate and difficult experiments of chemical laboratories in the con- 
fined space of their revolving domes. And if they did occasionally 
prepare “ wet-plates,” they had to use them immediately, or lose 
them; while if the sky happily remained clear enough for the 
using, — the wet-plate would not remain long in its highest state of 
sensitiveness, or cleanliness either ; for certain “ oyster-shell ” mark- 
ings, as well as “ fogging,” would begin to form upon it, pin-holes 
would multiply, and the image of a brilliant and minute point of 
light would begin to spread chemically in the film, until pungent 
star-points became, as to size, more like apples and oranges. 
But the new dry plates, after very easy purchase, can he kept 
waiting for an opportunity, through any length of time. They admit 
also of any length of exposure, without losing their sensitiveness, 
and without spreading the images of bright points so extravagantly 
as the wet films. And this ability of the dry-plates generally to 
stand long exposures is simply invaluable in photographing faint 
stars ; for, contrary to the human eye, which can either see such an 
