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Diopters of the 
1r 
Ipl 
+ 4 
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250 '''''^' 
■,' 
mti 
.35 
c 
800 mm. 
250 " 
m 
When in use the whole apparatus is placed upon the drawmg- 
, which serves as the source of transmitted -light, but reflected 
may be used equally well. 
One advantage of this camera is that, even with low powers, the 
field of vision is very large, so that objects from 6-10 cm. m 
diameter may be drawn. By placing the object in the place of the 
drawing, and the drawing in place of the object (using the above 
table), one can reduce the magnification from 1-^. 
While it is easy by means of the concave glasses to accommodate 
any eye to the instrument, the apparatus is also a safe and con- 
venient help in laboratories, even to the unexperienced. But as in 
all such instruments it is better to draw a ruler at the same time 
with the object. 
If one who is able to see clearly with the above combination by 
inserting at /a concave glass of —5 diopters, then, since the dis- 
tance of this glass from the convex lenses is 3.8 cm., the concave 
lens may be omitted and the convex lenses at b and d replaced by 
others, 6. 25 diopters smaller. With a concave glass of -}- 5 D. at /, it 
is possible to obtain a magnifying power of 8 times by inserting at 
d, 50 cm. from the object, a convex glass of + 20 D., and at ^ a 
convex glass of + 2,5 D., 400 mm. above the drawing-paper. If 
the concave at/ is omitted, then, leaving object and lens in same 
position as before, it will be sufficent to place at d & convex glass of 
+ 20 — 6.25=13.75 diopters, and at 5 one of + 2.5 — 6.25= — 3.75 
Ds. One may thus obtain an 8-fold power without using too 
strong glasses. For eyes of a different refractive index, the num- 
ber of diopters to be deducted changes. 
If in the previous combination it is necessary to have at / a con- 
cave glass of — 1 D., this may be removed by deducting 1 D. from 
the glasses at d and b. 
In the same way, 
t /, maj' be replaced by - 
!.16D's 
the first named combinati( 
concave glass ( 
