78 
that in this work we have the fulfilment of the remarks 
made by M. Janssen in the above quotation. 
The discovery of the nebula surrounding the star Maia 
in the Pleiades was a clear gain to science, and a demon- 
stration of the advantage of photography in mapping the 
heavens. It must be stated here that by prolonged exposure, 
and checking the clock-motion of the apparatus used by 
means of eye observation at the same time, M.M. Henry have 
achieved the almost perfect results I am enabled to show 
here this evening. The method employed is that- of using a 
photographic lens of large aperture and corresponding focal 
length, the plate used being about 11 x 9 inches, though 
why a square plate is not used I do not know. The almost 
true circular discs are obtained by correcting the clock 
during the exposure of the plate by means of a telescope in 
the same mounting as the camera directed to the same 
object and constantly watched; and as the plate marked 
No. 1 was exposed 3 hours (“ trois poses d’une heure”) the 
execution of the work needs the exercise of a large amount 
of patience, and in order to detect defects in the plates 
duplicate negatives are necessary. It need scarcely be said 
that the process employed in this case is the gelatine. For 
his solar work M. Janssen uses the collodion process, the 
extreme sensitiveness of the gelatine film would, for solar 
work, be a disadvantage. 
It is considered that with care and patience the whole 
heavens could be mapped by this means, and it is proposed 
that the work should be undertaken by a number of ob- 
servers using the same kind of apparatus. There may be 
some difficulties to overcome with the apparatus. A careful 
scrutiny of the photographs of portions of the constellation 
Cygnus shows that the stars in the corners of the plates are 
drawn out towards the centre, and this, no doubt, is caused 
by the lens not “ covering,” as it is termed, so large a flat 
surface — for the stars may be said to be on a flat surface for 
photographic purposes — and no doubt the full aperture of 
the lens is used, 
