195 
of Edinburgh, Session 1884 - 85 . 
to represent properly that one very small portion of the sky. So 
that an atlas, to show the whole sky on the same scale, would re- 
quire about 5 millions of them ; and by no means every photograph 
that is taken is always a success, and worthy of being kept; while 
every one that is kept requires at least two companions as good as 
itself, to guard against accidental imitations of little stars by either 
pin-holes in the film, or specks in the glass plate. 
And even if we should replace that hind of magnifying by the 
Achromatic compound microscope, — we are just as badly subject as 
ever to the inherent weakness of an originally small photograph, as 
distinguished from Nature herself further magnified, in this import- 
ant truth; viz., that we do not, by simply magnifying a discoloured 
film, separate close stars ; we merely enlarge their discs, or discous 
impressions. And at the rate of enlarging already indicated, such 
disc, or spurious photographic effect, in the case of any one of these 
three stars in the Belt of Orion, — would swell out into a huge circle, 
lio less than 3 ‘5 inches in diameter ! Utterly covering, concealing, 
or swallowing up therefore any interesting stellar companion such 
star might have, though it should be 800 times as far off as the small 
angular distances which astronomers have to deal with. 
While further still, though long exposure may bring out more 
stars than short exposure, with the same instrument, — it does not 
by any means enable a small telescope to compete with a large one 
in what it can show with any exposure. Photography of the stars 
therefore, though begun most meritoriously with small instruments, 
will have to be continued afterwards, in the accustomed ways of 
old, with larger ones. Larger ones possessing more light, and 
more magnifying power ; — but with the inevitable accompaniment 
of smaller angular fields of view ; and in that case there will 
ensue a great multiplication of the sensitive plates required. 
Wherefore in place of the telescopes of the future being, by photo- 
graphy, reduced to pocket size and minimum cost, — they will rather 
have to be made larger than ever, and worked more expensively. 
Hence it is that so ablo a practical astronomer as Mr Ainslie 
Common, who has been performing such wonders of astronomical 
photography with his grand reflecting telescope of 3 feet diameter 
of aperture, is casting about now for the erection of another tele- 
scope of 7 or 8 feet in diameter, or considerably larger than Lord 
