II.) Draw a d
the chord of the arc a c d. Trisect a d at o' and k. Produce
d a to l, making a l = a o' = o' k = k d. With a k as a
transverse axis, and l and d as foci, construct the branch of the
hyperbola k c c' c", which will intersect all arcs having the common
chord a d at c, c', c", etc., making the arcs c d, c' d, c"
d, etc., respectively, equal to one-third of the arcs a c d, a c' d,
a c" d, etc.
* * * * *
TEST CARD HINTS.
By Dr. F. OGDEN STOUT.
I know it is the custom with a great many if not the majority of
opticians to fit a customer without knowing whether he has presbyopia,
hypermetropia, or any of the other errors of refraction. Their method
is first to try a convex, and if this does not improve, a concave,
etc., until the proper one is found. This, of course, amounts to the
same thing if the right glass is found. But in practice it will be
found both time saving and more satisfactory to first decide with what
error you have to deal. It is very simple, and, where you have no
other means of diagnosing (such as the ophthalmoscope), it does away
with the necessity of trying so many lenses before the proper one is
found.
Pages:
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51