322 Coleman Sellers on the Globe Lens 
size, when reproducing the size of the original. As the lenses 
increase in size and length of focus, the plates covered increase 
in size, and the amount of glass in the lenses bear a larger pro 
portion to the brass work in which they are mounted, and hence 
the included angle of vision is increased, so that while in the 
24 inch globe the included angle is 75°, in a 12 inch globe (that 
is, one of 12” focus) the included angle is over 90°. It has been 
said that the light, being admitted through a round hole m4 
plane plate in the centre of the instrament, must be muc more 
intense at the centre of the field than at the margin, and some 
writers have stated this fault to be one of great magnitude. 
Practice however does not show the evil to be so great as they 
represent, if it exists at all. By reference to the cut, it W be 
seen, that the dotted lines « representing a pencil of light of the 
diameter of the centre opening passing through the axis of ™ 
instrument, and y y representing a pencil of light passing thr 
the same opening obliquely, doubtless the area of the centre one 
will be the largest, but as it passes through much thicker a Ae 
than the rays yy, may not the ultimate effect of each be equ 
ized? I do not pretend to any knowledge of the theory of of : 
tics, and must confine myself to facts. In the trial of many va 
these lenses, of different sizes, I have never found the err 
exist, and all the pictures I have made with the globe ~ : 
are remarkable for the even illumination of the field. 
ae 
the last two meetings of the Photographie Society of Mie ae 
— (February, 1863,) the merits of these lenses have D&™ ne 
iscussed—full credit for correctness of reproduction was ac 
corded to them by all; but the quickness of working wright he 
ight B 
tioned b tl ho stated that in broad sun i a 
10n 'y one gen eman, who sta atin oro oe obit ! 1 
had exposed an engraving for several minutes and h 
an under-exposed negative, while all others present who 
tried them were unhesitating in their assertions 
remarkably quick workers when the size of aperture 
into consideration. A few days ago I placed in bright 
an engraving from the London Art Journal, and cople 
leman who questioned its quickness. 
oper, assuming great intensity in the light and shOW! © 
Kable symptoms of over exposure, so that I can beat! 
that they were - 
was taken 
jon® | 
igs ee a 
ot plate with the same size instrument as was use@ - of 95 : 
exposure @ ae 
ve an impression which flashed up instantly ing 
