Andrew Jay Cross.

A system of ocular skiametry including such portions of optometry as are pertinent to the use of the shadow test with the plane mirror online

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Online LibraryAndrew Jay CrossA system of ocular skiametry including such portions of optometry as are pertinent to the use of the shadow test with the plane mirror → online text (page 1 of 11)
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A System of Ocular Skiametry





Author of " Dynamic Skiascopy, or the Action of the Accommodation in Shadow Testing,

and other papers. Member of the Optical Society of the City of New York

President of the American Association of Opticians, 1900 to 1901

President of the Optical Society of the State of New York

1897 to 1900. Honorary Member of the Central

New York Optical Society and of the

Rochester N. Y. Optical Club

etc. etc.

IVith Ninety-Four Illustrations



36 Maiden Lane








Those Opticians

Who Have Shown Themselves Ambitious to Elevate Their Calling

This Book is Fraternally Dedicated.



It is with some timidity, both from a literary viewpoint
and from tliat of a retailer of spectacles and eyeglasses, and
manufacturer of optometrical devices, that this book is offered
for the consideration of those who adapt glasses to the eyes
of others.

If it is true that "necessity is the mother of invention,"
then perhaps it will be idle to offer further excuses here. The
author, however, desires to express his deep appreciation of
the work done by the many medical men who have enhanced
the value of Sir William Paget Bowman's discovery of the
phenomena now known as the "Shadow Test." Especially is
the author under obligations to such writers as Dr. Edward
Jackson and Dr. James Thorington for aid derived from their
able monographs on this subject. But nearly all medical
writers having, naturally, written from the standpoint of
physicians, that "the use of a reliable cycloplegic is always
indicated," it has remained for opticians to devise ways and
means for accurately applying the principles of this test with-
out resorting to the use of local toxicants of any kind.

This small volume has therefore been prepared for the
purpose of pointing out new and better paths in practical
optometry, as well as the re-blazing of old ones, and the
author begs that all optical and medical readers, particularly
those who are wedded to old-fashioned methods, will bear in
mind that the use of mechanical devices is the order of the
day, and that the accuracy of the experiments alluded to in
the following pages can be readily proved by any one suf-
ficiently interested to attempt their demonstration.
20 East 230 Street, New York.
October i, 1(^03.

Dbraryof the Alamerfa

Counuy. Assoeiation

of OpU^metrists


CH APT KR I PaRcs 1 1 to 2^.

A Descriptive Name.

Amount of Optical Knowledge Necessary to Achieve Skiametric

The Value of Bowman's Discovery.
Difficulties of Ocular Skiametry.

CHAPTER II Pages 24 to 49-

Adequate and Inado(iuate Examination-Rooms.

Illumination: Its Size, Source and Control.

The Plane Mirror: Its Construction and How to Handle It.

Good and Bad Schematic Eyes, and How to Correct Them.

A Short Method for the Reduction and Transposition of Lens


CHAPTER III Pages 50 to 69.

Optical Principles Involved in Skiametry.

The Shadow, What to Look For and How to See It, and How to

Imitate Its Action with a Card-board Model.

CHAPTER IV Pages 70 to 86.

.•\ction of the Shadow and What It Indicates.

Appearance of the Sliadow in Regular and Irregular Errors of

Refraction and Its Action in Emmetropia, Myopia, Hyper-

metropia and Astigmatism.

CHAPTER V Pages 87 to 97.

Some Theories Regarding the Dulness of the Fundus Reflex in

Certain Cases.
Also Multiple Methods of Practising Skiametry, Including the

Toxic and Non-Toxic Planner of Employing the Static Test

by l-5()tli the .Amplifying and Fogging Methods.

CHAPTER VI Pages 98 to iii.

Dynamic Skiametry and Its Uses in Mastering Tonic and Clonic

The Importance of Visual Fixation in Obtaining Bright Retinal

Reflexes, and the Aid Derived from Using Independent Points,

Together with a Few Words .About Ray Values.

CONTENTS— Continued

CHAPTER VII Pages 112 to 123.

Ocular Muscle Action, and the Influence of Habit Upon Accom-
modation, Convergence and Innervation, with Special Reference
to Spasms, Latent Errors, and the Use of Prisms.

CHAPTER Vni Pages 124 to 129.

Corroborative IMeasurements in Optometry and the Inter-Depend-
ence of Objective and Subjective Methods.— The Value of
Mechanical Devices and the Superiority of the Mobile over
the Unit Action of Lenses.

CHAPTER IX Pages 130 to 146.

The Use of Instruments in Skiametric Work, Their Evolution
from Single Lenses and the Relative Merits of Some Now
Employed, including a Description of the Author's Own
Mechanical Contributions in this Line.

CHAPTER X Pages 147 to 153.

Systematic Ocular Examinations and the Aid Derived from Mak-
ing Complete Prime Records. — Resourcefulness in Refraction
Work and the Successful Exainination of the Eyes of Children,
Mutes and Illiterates.

CHAPTER XI Pages 154 to 168.

Illustrative Cases, Showing the Comparative Value of Static and
Dynamic Skiametry in Various Patients of Different Ages,
Occupations and Apparent Physical Condition.

CHAPTER XH Pages 169 to 181.

Resume of Previous Chapters With a View to Emphasizing the
Salient Points of Ocular Skiametry as a System.


Fig. Page

1 Parallel rays of light being focused 15

2 Fociised rays of light being paralleled 15

3 A "Success" burner oil lamp 27

4 An "Argand" burner gas lamp 28

5 An acetylene gas lamp 29

6 A "Welsbach" gas lamp 30

7 A "Welsbach" gas lamp with a large asbestos-lined "Cross"

chimney 31

8 A small asbestos-lined "Cross" chimney for citlier a "Welsbach"

or an "Argand" lamp 32

9 A gasolene "Student" lamp x^

10 Showing spiral filament for electric lamp 34

11 The "Cross" asbestos-covered electric lamp 35

12 The "DeZeng" luminous retinoscope 36

13 Wall bracket for gas or electric lamp 37

14 Section of the "Cross" mirror showing concave l)ack 39

15 The "Cross" Bracket ]\Iirror 40

16 Showing manner of holding mirror 41

17 A "Queen" pasteboard schematic eye 43

18 A home-made "smoke" box 51

19 Showing refraction by lens of proper focus 51

20 Showing refraction by too strong a convex lens 5-2

21 Showing refraction by too weak a convex lens 52

22 Rays of light emerging unrcfracted 54

23 Rays emerging properly refracted 54

24 Emerging rays having a conjugate focus 55

25 Rays emerging divergent 55

26 Rays emerging unrefracted 56

27 Rays emerging insufficiently refracted 56

28 Rays emerging properly refracted 57

29 Rays emerging with divergency increased 57

30 An Emmetropic eye 5^

31 A Myopic eye 59

32 A Hypermetropic eye 59

33 Illustrating a so-called "shadow" 61

34 Showing now the retma is illuminated l)y reflected light 63

35 Showing the source of the returning light to be the edge of

the illumination 63

36 First half of pasteboard model for demonstrating the shadow's

action 65

f'^- Page

37 Second half of pasteboard model for demonstrating the

shadow's action g^

38 Showing why the shadow moves with the mirror 71

39 Showing why the shadow moves against the mirror jj

40 Illustrating total refraction in Emmetropia j;^

41 Ilhistrating a weak degree of Hypermetropia 74

4-' Illustrating a marked degree of Hypermetropia 74

43 Illustrating a weak degree of Myopia 75

44 Illustrating a marked degree of Myopia 75

45 Illustrating Hypermetropic Astigmatism 76

46 Illustrating ^Myopic Astigmatism -(]

47 Illustrating Compound Hypermetropic Astigmatism j'p

48 Illustrating Compound Myopic Astigmatism ^^

49 Illustrating Mixed Astigmatism yg

50 Illustrating Mixed Astigmatism at oblique axes 78

51 Showing the crescent-like shadow in Spherical Cases 81

52 Showing the straight-edge appearance of the shadow in

Astigmatic Cases 81

53 Showing the band observable in high degrees of Astigmatism. . 82

54 Showing an Astigmatic band at oblique axis 82

55 Showing the meridional appearance of the shadow in Compound

Astigmatism 83

56 Showing the double band in the "Scissors" movement 84

57 Showing a case of "Irregular" Astigmatism 84

58 Showing a case where "Cortical Cataract" is present 85

59 Showing the shadow as it sometimes appears in "Conical

Cornea" 85

60 Showing why the retinal illumination is large in marked errors. 88

61 Showing why the shadow moves slowly in marked errors 89

62 Showing why the shadow is duller in Myopia than in a like

degree of Hypermetropia go

63 Showing relative size of retinal illumination in high and low

degrees of Myopia 91

64 Showing the optical principles of Penumbra 91

65 Showing optical principles of Penumbra doubled 92

66 Showing the interference of Penumbra in shadow-testing 93

67 Showing emerging rays being bent to a focus by a trial lens. ... 100

68 Shov/ing emerging rays being bent to a focus by accommodation loi

69 Showing how accommodation can be made to absorb a ciliary

spasm 103

Fig. Page

70 Showing the assistance offered by niuUiplc fixation points in

Dynamic Skiametry 106

71 "The Cross" Fixation Stand 107

^2 Showing primary position for fixation card 108

73 Showing normal balance between Accommodation and Con-

vergence in Emmetropia 113

74 Showing relative Innervation necessary to balance Accommo-

dation and Convergence in Emmetropia 114

75 Showing lack of balance between Accommodation and Con-

vergence in Hypermetropia 115

76 Showing the unequal Innervation required to balance Accom-

dation and Convergence in Hypermetropia 115

'•J Showing lack of balance between Accommodation and Con-
vergence in Myopia 1 16

78 Showing the unequal Innervation required to balance Accom-

modation and Convergence in Myopia n6

79 Showing the optical principles of mobile lens action produced

by changing the relative position of lenses 128

80 The skiascopic lens rack of Wiirdemann 131

Si The disc form of holding lenses, used by Crain, Standart

and others 132

82 The constructive principle of the "Jennings" instrument 133

83 The constructive principle of the "Meyrowitz" instrument 134

84 The constructive principle of the "Fay" instrument 134

85 The constructive principle of the instrument designed and used

by the author in 1892 135

86 The constructive principle of the "Geneva" instrument pat-

ented about 1898 136

87 The constructive principle of the "Cross" Retino-Skiameter 138

88 External appearance of the "Cross" instrument 140

89 Showing how to alter pupillary distance 141

90 Showing manner of using instrument 142

91 Showing instrument in dust-proof case 143

92 Showing comparative size of normal and magnified pupil 146

93 Sample record blank 148

94 Manner of filing records I49

A System of Ocular Skiametry


A Descriptu'e Name. — Amount of Optical Knowledge
Necessary to Achieve Skiamf.tric Success. — The
Value of Bowman's Discovery. — Difficulties of Ocu-
lar Skiametrv.

A DESCRIPTIVE NAME. It is a very doubtful ques-
tion whether any method for examining an eye, in any manner
whatsoever, can be given as many names, each with as plausi-
ble a relevancy as those already given to the phenomena of
light and shadow seen in the pupil of an eye. These phe-
nomena having been first described by Sir William Paget
Bowman some forty years ago, and later known as the ''Sha-
dow test."

Since the introduction of the blanket word "Optometry."
to cover all measurements of an eye whether external, internal
or functional, the suffix "metry" seems to have been especially
assigned to duty in connection with those ocular conditions
wherein it is possible to find a variance from certain fixed
standards. Thus the names Ophthalmometry, Dioptometry,
Astigmometry, Keratometry, Phorometry, Strabismometry. Pu-
pillometry. Perioptometry, and Visuometry are in use. Then
if the lines are drawn a little finer there are found such names
as Exophthalmometry, Ophthalmotometry, Ophthalmotropo-
metry, Chromotometry, and. in the measuring of lenses and
prisms, there are also the words Phacometry and Prisopto-

The Greek derivative "scope" (sight, or examination),
'.eems to have been closely allied in the past with shadow-test-


ing, and the equivalent of such words as shadow-seeing, retina-
seeing, pupil-seeing and others with visual reference have been

The Greek words for '•seeing" and for "measurin


sometimes construed as opposites, but the English word for
"measure" seems to be sharply defined : "To compare with a
fixed standard." Now, in shadow-testing this is exacth- what
an optometrical examiner really does; he measures an eye by
means of reflected light, together with known ray-bending ap-
pliances called lenses, and compares his finding's with a cer-
tain fixed standard.

It is common to speak of measuring time, and it is known
that light, shadow and sundials were formerly employed for
this purpose. From this to measuring passing shadows is not
a long stretch of the imagination, and as an examiner must be
informed regarding the direction, speed and volume of all
ocular shadows in ascertaining the relation of the ray-bending
power of an eye to its retina, it would seem that this part
of a refractionist'.s work comes nearer to being cye-shadow-
measuring than it docs to anything else.

The plea that the word ".Skia."' for shadow, should be
omitted from the term used to denote the shadow test does
not seem a logical one; for from first to last an examiner is
endeavoring to ascertain the action of the shadow, not of the
retina, by the changing of lenses in front of an eve for the
purpose of measuring the angle of the emergent ra\s. There-
fore it follows that the word, "Retinoscopy," though an old
one, is not necessarily a good one, because "seeing the retina"
in this sense is really not any more descriptive of that which
is accomplished than would be the same name if applied to
that which is now called "Ophthalmoscopy."

The term "Skiascopy" seems a better one than that of
"Retinoscopy," although seeing-the-shadow is not verv de-


scriptive either, unless it be preceded by the general eye term
"ocular,'"' when it could be translated "Eye-shadow-seeing."

It has been urged as an objection to the use of the word
"skia" that it might lead to confusion with the pictures, or
skiagraphs, made by the X-rays of Roentgen. In answer to
this it may be said that the nomenclature of optical science
is already so complex that technical terms must be given
sufficient length in order to convey their proper meaning,
otherwise misunderstanding and confusion between teacher
and student will ever be on the increase. And so it behooves
the ocular-shadow-measurer and the shadowgraph expert to
each look to his own descriptions in order that they may meet
intelligent requirements.

The device which reflects the light into an eye and enables
an examiner to see the shadow ought logically to be called
a shadow-viewer, or skiascope. And the instrument or
mechanism which permits of the measuring of the angle under
which a shadow is seen might correspondingly be termed a
shadow-measure, or skiameter, while the act or process of
measuring the behavior of the shadow in an eye could be
given the technical name of ocular skiametry. Especially is
this term applicable, since, as will be seen later on, it is now
quite necessary to qualify and subqualify this term in order to
represent gradations of meaning in the exact description of
Bowman's great discovery, no longer as a single method, but
as a system for examining and measuring the eye in many

general optical principle of the shadow test in its simplest
form is not very complicated. Taken, however, in connection
with its optometrical associates it represents as a whole a
rather high order of knowledge regarding both physical and
phvsiological optics. It also implies in its application a cer-


tain amount of skill or dexterity in the manipulation of indis-
])ensable mechanical devices, such as the skiascope and ski-
ameter, no matter whether the latter be a simple trial frame
with test lenses, or a more elaborate and useful apparatus.

There are two leading accomplishments in shadow-testing
in which an examiner must be proficient before he can achieve
success in this work. The first of these is the control of the
reflected light and the determination of the direction of the
shadow's motion under both favorable and unfavorable con-
d ition s. The^ second lies in being able to add and subtract
known refractive lens quantities and to tell with precision
what their ray-bending value is at all distances from an eye
under examination. To express it tersely then, an examiner
must be able to detect any action of the shadow and to know
exactly what the optical value of this action is when influenced
by either lenses or accommodation.

The first of the above requirements can be gained by dailv
practice, but the secorid requires considerable study and appli-
cation, as it involves a knowledge of angles of light, or ray
values, as well as of refraction, or lens values.

When a patient's eye is considered as an object, instead of
as a Subject, then its refractive condition must be determined
by noting the behavior of the light reflected from the retina
as it leaves the eye, and methods of procedure known as "ob-
jective" must therefore be applied.

Many students of optics who have confined their efforts
to a mastery of "subjective" optometry find themselves quite
at sea when they undertake objective methods. And the
reason for this usually lies in the fact that they have given
attention to the subject of light as it travels in one direction
only, namely, as it enters an eye.

One of the foundation principles taught in optical text-
books is that light returns over the same course which it has


traveled. Hence, if parallel rays of lij^^ht are made to pasf
through a convex lens they will come to a focus at the so
called "strength" of the lens. Invert this order, by placing-
a lighted candle at the focus of the lens, and the rays of ligh»
will diverge until they pass through the lens, after which
they will be parallel again. See Figs, i and 2.

Fig. I


parallel rays of light ueixg focused.
Fig. 2.

__ ^


In shadow-testing the retina of an eye is the apparent
source of light, although in reality the retina is only a noor
quality of mirror which reflects the light thrown into an
eye by the skiascope. This illumination, or reflection, behaves
like a piece of red flannel, or any other visible object which
acts as a high or low grade mirror according to its quality,
or ability, to reflect light, glass with amalgam backing, and
polished metals, being of the highest order, while lampblack
and black velvet are of the lowest.

The corelation of accommodation and convergence is an-
other subject which students of ocular skiametry must under-
stand in order to do their work intelligently. Thus it will be
seen that skiametric proficiency involves a pretty thorough


grounding in something more than rudimentary optics. With
the ehmination of the use of the concave mirror, however, and
by the aid of modern apparatus it is now possible to dispense
with many details which formerly resulted in the confusion
of beginners. Still, notwithstanding this, a student will find
much that will call forth his best efiforts before he can feel
assured of the reliability of his findings.

It is one thing to master ocular skiametry under regular
conditions and quite another to rightly differentiate the irregu-
lar and apply that judgment which secures success. But, as
in other studies, the deeper the student delves the more he
finds to learn, and the easier do the foundation principles
become. The wise searcher after the optical facts which in
the aggregate constitute optical knowledge will first learn the^
A, B, C of light and lenses, together with the reduction and
transposition of the latter, and next he will master physiological
optics, in addition to the art of subjectively correcting with
lenses any manifest conditions which may be met. The stu-
dent may then be said to possess sufficient optical knowledge
to begin the study of ocular skiametry with a fair chance of
achieving success.

Many inquirers into the merits of shadow-testing seem to ^
possessed with the idea that every case which presents itseK
is capable of being both easily and accurately refracted by
means of Bowman's discovery. This expectation is as in-
consistent with the real facts as it would be to expect like
results from trial case tests or any other one optomctrical

The truth can, perhaps, be fairly expressed by saying that
shadow-testing bears to trial case testing much the same rela-
tion as the addition of a column of figures from the top bears
to its addition from the bottom.


Skiascopy will uncover at a single sitting optical condi-
tions which it would be quite impossible for ordinary trial
case tests to do. On the other hand, the latter will show
visual conditions of which the former can tell nothing.
\'iewed again from a similar position we find that the two
general methods for estimating ocular errors of refraction,
known by the terms "objective" and "subjective," are like
seeing for one's self and taking the testimony of others.
Usually either method is fairly reliable in ordinary cases, but
in extraordinary ones — the kind that make and break reputa-
tions — the evidence can be none too corroborative.

So we find skiascopic and trial case-testing to be inter-
dependent, both systems having their weak and strong points
and one aiding in the judgment requisite for the successful
application of the other, skiascopy coming first because it
is the great refractive pilot, or pathfinder, and because, too.
there are many conditions other than errors of refraction
that are shown up by its use, and which, if it were not for
this early use, might needlessly prolong an otherwise short

It is hoped, therefore, that this point is made clear regard-
ing the value of shadow-testing. It is deemed of no more, nor
less, value than the trial case test, and that neither one is
infallible, and that both are absolutely essential in all prime
cases, whether the results obtained by either coincide with
those of the other or not, for this very lack of coincidence
is often the key which enables a trained judgment to solve
a refractive riddle.

By basing their judgment upon the principle that the
proof of the pudding lies in interviewing him who has chewed
the string, some credulous inquirers have been led to estimate
the merits of shadow-testing by taking the testimony of those
who have falsely pretended to possess a thorough knowledge


of it, and this unreliable information has led them to believe
that if skiametry is faulty in some hands it must be faulty
in all. Whereas the reverse reasoning would in all probability
be productive of better results, for that which one can achieve
by study and practice it is (|uite possible for others to achieve
by equal application and efifort, and sometimes by even less
where assistance is given by skilled tutors.

For nearly four decades the ablest optometrical researchers
have striven their utmost to find a better objective means than
skiametry for determining the optical condition of eyes, but
so far without avail. And judging from the present advanced
knowledge regarding optics and optometr_\- it is pretty safe
to say that the shadow test is here to stay, for a long time
at least, and that those whose duty it is to adapt glasses to
the eyes of others will find their work more reliable and
much easier if they take the time to thoroughly master this
valuable means for ascertaining ocular refractive conditions
in a manner independent of the patient's intelligence.

Xow this phrase, "independent of the patient's intelligence,"
may prove somewhat misleading, since even those who are
experienced in skiametric work find many cases in which the
results obtained are very unsatisfactory indeed. Yet, wlion
an examiner measures a case by skiametry and notes an error

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Online LibraryAndrew Jay CrossA system of ocular skiametry including such portions of optometry as are pertinent to the use of the shadow test with the plane mirror → online text (page 1 of 11)