PJiotograjjhy an Aid to Astronomy. 
229 
(2) The photographs themselves form a permanent record of what is 
seen more reUable than can be obtained from any drawing or description 
executed at the hands of man. 
The early applications of the art of photography to astronomy related 
to the subject from its descriptive aspect rather than to those branches 
of the science which depend on exact measurement. With the aid of 
reflecting telescopes of large aperture it was found possible to delineate 
with certainty the forms of vast nebulae, so as to establish with great 
nicety the extension of their convolutions into regions where to the eye 
alone their traces were quite invisible, even with the aid of powerful 
telescopes. But fascinating as these photographs are in aspect, valuable 
as they are as early records, and suggestive as they are with regard to 
the cosmical significance of the objects photographed, it can hardly be 
maintained that they have so far been prolific in scientific results. Such 
phenomena as the wonderful spiral forms assumed by many of the 
nebulae, though proved to be far more common than was previously 
realised, remain as obscure as ever in their origin. 
In making this statement I do not wish to underrate the work of the 
earlier pioneers in astronomical photography. They have developed 
methods which we continue to use and have but little to improve on, 
and they are not to be blamed if the full value of their work has not 
been attained in their own generation. Visual work had already been 
carried so far that the extended vision afforded by photography could 
hardly have been expected to lead instantaneously to any new and 
startling phenomenon which should revolutionise existing ideas of the 
extra-terrestrial universe. It is only as time lapses and we are able to 
study the changes which are taking place since these photographs have 
been secured — changes which as a rule take place so exceedingly slowly 
as not yet to be fully established — that the full advantage of the new 
method over the old will be realised. 
The earliest attempt at the photography of celestial objects seems to 
date back as far as the year 1840, when photographs of the moon were 
obtained by daguerrotype processes, but beyond its use for pictorial 
presentation — to which perhaps I might add the daily photographic 
record of the sun's surface instituted at Greenwich in 1873, and unsuc- 
cessful attempts by Pritchard at the instigation of Warren de la Eue to 
utihse it for the determination of the constant of nutation — for a long time 
it seems to have been regarded with suspicion for purposes requiring the 
use of exact measurements. While photography might supplant the 
astronomical draughtsman, its limitations were in other respects similar 
to those affecting the draughtsman's art and skill. It is its uses in con- 
nection with precise measurement with which I propose chiefly to deal 
to-night; not that I wish to underrate the importance of other applications. 
