320 Coleman Sellers on the Globe Lens 
the art isconcerned. The negatives of to-day look like the nega 
tives of the first experimenters, and the chemical process of their 
roduction is essentially the same. But with the optics of pho- 
tography the case is ditferent—here there has been a steady im- 
provement. The wants of the portraitists have been met by the 
construction of new objectives suited to the style of pictures to 
be produced. In these instruments depth of field with free ad- 
mission of a large volume of light was what was most sought for. 
Theory could not dictate what shape or combination of lenses 
would best produce this result, and patient experiments were Te 
sorted to. ‘The requirements of landscape photography are quite 
different from those of portraiture. A portrait tube may be used 
to take views if it be provided with a stop or small opening to 
limit the amount of rays passing through it and thus to deepen the 
field, or increase the ‘reach’ of the instrument as it is technically 
called. This involves loss of light, and consequently diminishes 
the quickness of its working. We hear continually of rapid or 
instantaneous photography, and are often led to believe that the 
rapidity is to be ascribed to some wonderful sensibility of the 
chemicals used, but this is only partially true, and to the optician 
is due the most of the merit of instantaneous pictures. te 
trait tube with its full opening will, in a sky-light room, proauce 
a picture in perhaps ten or fifteen seconds. This same inst? 
ment, with the same opening and same chemicals, exposed to #8 
extended view in bright sun light, could not be opened and shut 
arge an area of space being concentrated on the same — 
not be brought into focus at the same time. a 
The human eye, when the head is at rest, takes in an angle 
of view of at least 70° or 80°, the whole of which is not see 
clearly at once but can be examined in detail by the almost UF 
conscious rolling motion of the eye in its socket—the arnt oS 
Hence a picture of a landscape, for instance, to fill the eye a 
seem a true representation of nature, should include an angle 
at least 60° 6 
