The physics and philosophy of the senses;

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Preface

The subject of animal life, and the connection between the mind and the external world, is one of the most interesting and mysterious which can occupy the attention of the curious.

Within the last twenty years, by the skilful employment of the microscope, the subject in its physical bearings has had a large amount of additional light thrown on it, and histology, by its numerous triumphs, has risen to the dignity of a new and highly important science.

Even within the last few years discoveries have been made which have materially affected the commonly received theory of vision, and have shown that our views regarding the functions of the principal parts of the organ of sight have been resting on a misconception.

These circumstances would alone be sufficient to justify an attempt to popularize the subject of the senses. But there are other more important considerations which have influenced the author in writing the present volume.

The author has had two distinct though closely allied objects in view. First, to supply a popular manual of the

VIU PREFACE.

senses, which, as regards the physics and physiology of the subject, might embrace the more important discoveries of recent times, and might thus be useful to the reading and inquiring portion of the community. Second, to present, for the consideration of competent and independent thinkers, a philosophical treatment of the subject, which, it is generally admitted, is far from having yet acquired that consistency and completeness which its importance has pre-eminently deserved.

In discussing the subject of life, organisation, sensation, and thought, the author, in opposition to the materialistic theory, has endeavoured to demonstrate that the senses, no less than our reason, furnish proof that an immaterial and spiritual element is the operative element in nature. The subject of power, force, or energy, its origin, its modes of working, and its transmission, has proved during the last thirty years one of the most interesting which science has handled, and it has yielded, perhaps, more solid fruits than any other branch of physical science.

Power, or force, being at once the cause and the measure of every physical change, we may regard it in this light, as affording us a key to the right apprehension of every branch of physical philosophy.

In Parts I. and VI. the author states his reasons for adopting a dynamical theory as the true foundation of philosophy ; and in its proper place he shows that this theory has been held by men of the highest eminence in science and philosophy, and that it is becoming in our day more and more an article of well-founded scientific conjecture and belief. It is generally admitted that we have no knowledge of matter as an entity, or thing in itself : all we perceive in physical

PREFACE. IX

phenomena are various manifestations of localized force. Philosophy, in such circumstances, has no choice — it must found on what it knows, and not on what it imagines.

The advantages of such a theory are, that while it allows the utmost freedom in the prosecution of physical inquiry, it at the same time satisfies philosophy. It furnishes an exhaustive theory of perception by removing the difficulties which since the days of Descartes have obstructed the progress of philosophic realism. Nor is it one of its least merits that it serves materially to elevate our conceptions of the dignity of the physical world, by leading us to regard it as a system specially designed and arranged for the benefit of sentient beings, and for regulating their intercourse with external nature.

More than twenty-two years ago the author, rather for his own amusement than with any serious object, published a volume entitled " The Philosophy of the Senses.*' The subject which he thus rashly undertook to handle was, at the time he entered upon it, one almost entirely new to him. As he proceeded, however, it opened out before him with its rich variety of facts, ideas, and speculations ; especially when it came to be considered in its relation with mental phenomena, and with the multitude of curious metaphysical questions which these naturally suggested.

Since the period alluded to, the subject so lightly entered upon has continued to engross a considerable portion of the author's attention, and now, with leisure at his disposal, and with much more definite aims, he embodies in the present volume the results of more extended knowledge and reflection.

Except in its general arrangement, and in certain pages

X PREFACE.

of the chapters which treat of the physical laws of sound and light, and of taste and smell, this is an entirely new work.

For convenience, the author has divided the volume into six parts.

Part I. treats of Matter, Organisation, and Animal Life. Part II. treats of the Laws of Sound and Light, and the Organs of Sense. Part III. treats of the Special Senses, and of Touch, or General Sensibility. Part IV. treats of the Nervous System. Part V. is a Critical Sketch of certain leading Philosophical Works, Opinions, and Speculations- Part VI. is an attempt to explain and illustrate the Philosophy of the Senses.

Edinburgh, December, 1874. »

Part I. Matter- Organisation— Animal Life, Etc

CHAP. PAGE

I. Matter— Its Atoms — Their Energies. Power— Our Conceptions of it ... ... ... ... ••• 3

III. Animal Life and Endowment ... ... ... ... 3^

IV. The Genesis of Animal Life and Organisation— Specula-

tions— Transmitted Life — Transmitted Mind — Habits,

ETC. — Tyndall's Views, Huxley's Views ... ... 52

V. Sensibility, Sensation, and Thought ... ... ... 62

VI. The Development Theory and the Doctrinf of Design —

The Natural and the Supernatural in Nature ... 76

Part II. Physical Laws And Physical Organs

Human Knowledge — Its Nature — Its Limitation — Inorganic Agencies the Media of our Sense- Knowledge — Sound — Atmospheric Elasticity — Tension — Waves of Condensation — Of Rarification — Propagation and Diffusion of Sound — Dr. Young's Experimaits.

Range of the Hearing Faculty — Pitch and Quality of Sound. IX. Sound {^Continued) ... ... ... ... ... iii

Harmony — A Note — Nature of Waves produced — Types of Waves of Equal and Unequal Lengths — In Combination — Concords — Beats — Natural Harmonics — Scales of Instruments without Finger Keys — The Diatonic Scale. X. Sound {Continued) ... ... ... ... ... 124

Speech — Its Intonations — The Ear — Its Structure — Recent Discoveries.

Xll CONTENTS.

Newton's Corpuscular Theory of Light — Undulatory Theory — Colour of Bodies, etc.

Radiation of Light — Decrease of Intensity — Apparent Brightness— Suppositional Modes of Vision.

Refraction of Light in Parallel and in Triangular-sided Prisms — The Solar Spectrum : Modern Discoveries — Ratio of the Sines — Lenses — Chromatic and Spherical Aberration — Helmholtz on Defects of the Eye.

The Human Eye — The Functions of the Retina — Modern Discoveries.

Size of Objects — Natural Process of Magnifying — Artificial Means — By an Artificial Pupil — By the Simple Lens, in two Ways — By Combinations of Lenses.

The Appendages of the Eye.

Heat and Light— The Ethereal Medium— The Diffusion and Transmission of Power.

PART IIL

THE SPECIAL SENSES AND GENERAL SENSIBILITY.

XVIII. Theory of Vision ... ... ... ... ... 213

Difficulties Connected with the Theory of Vision — Single and Double Vision and the Doctrine of the Corresponding Points of the Retin£e explained — The Use of Two Eyes.

XIX. Theory of Vision {Continued) ... ... ... ... 229

Visual Direction — Inverted Retinal Image and Erect Vision — Misconception as to Visual Direction.

XX. Theory of Vision {Continued) ... ... ... ... 242

Ocular Spectra — Physiological Conditions of the Retina — Coloured Spectra — Harmony of Colours — Irradiation of Sensation — How its Disturbing Effects are Obviated.

Its Vindication — Its Organ — The Tongue — Its Various Offices — Its Papillae — Cravings of this Sense — Its Effects on the Human Race — Extension of Man's Bill of Fare — Its Unequal Progress — Classification of Tastes.

CONTENTS.

XIU

CHAP.

XXII. Smell

Its Office — Its Field— Principle of Repulsion exhibited in the Molecules of Bodies — Smell conversant with these Molecules — Its Range of Perception — Its Nerve — Its Offices— Its Suggestive Powers — Associated with the Ludicrous.

XXIII. Touch

How many Senses are there? — Different Modes of Classification— Difficulties with regard to the Sensations of Touch — Taste and Smell frequently confounded — Sensations Infinite in Number — Sympathy between the Body and the Mind— Vital Energy — Physical Depression— Philosophy of Pain — Control over Nervous Energy.

XXIV. Touch {Continued)

The Skin — Its Sensitive Papillae— Its Glands — Law of Decreasing Sensibility — Its Discriminating Power at Different Parts — Distinctions in Sensations interpreted by Experience — Misinterpreted — Modified by Attention — Pain Modified by Resolution — Ideal Pains — Pains from Morbid State of the Nervous System — Nervous Effects of Animal Magnetism — Spiritualism — Chloroform — Tickling — The Drop — Homoeopathy — Reflections on the Nervous Sensibility — The Velocity of the Transmission of Sensation and Volition calculated — The Brain proved to be the Sensorium — Is it the Percipient Principle? — A Moral Reflection.

PAGE

Part IV. The Nervous System

XXV. The Nervous System

The Sympathetic and the Cerebro- Spinal Nerves. XXVI. The Nervous System {Continued) ...

The Brain — What we know of it — The Principle of Brain Action, or its Function as a Physical Machine — Its Parts and Connections — Its Histological Structure — Functions of its Different Parts.

XXVII. The Nervous System {Continued) ... The Cranial or Cerebral Nerves.

XXVIII. The Nervous System {Continued)

Functions of the Nervous System — The Sympathetic Nerves — Reflex Action — Consensual or Sensori- Reflex Action — Voluntary Motion — Animal Intelligence — Mental Action with Apparent Unconsciousness — Difficulty of acquiring New Movements — Ultimate Facility — Explanation — How far Man is a Machine — The Directing Power.

XIV

CONTENTS.

Part V

CRITICAL SKETCH OF SOME LEADING PHILOSOPHICAL OPINIONS AND SPECULATIONS.

CHAP.

XXIX.

XXX.

XXXL

XXXII.

XXXIII.

XXXIV.

XXXV.

Introductory Remarks

Plato.

Succeeding Schools

Neo-Platonism — Philo — Plotinus- Scholastic Philosophy.

Rise of Modern Philosophy

Descartes.

Difficulties Arise

Geulinx — Malebranche — Spinosa- Locke

Berkeley ...

Kant

-Proclus — Patristic and

-Leibnitz.

PAGE

347 361

370 379

392 426

Part VI. The Philosophy Of The Senses

XXXVI. Introductory Remarks on the Scottish School.

Reid on the Direct Perception of the World — Hamilton on this Subject — Plis Views on Solidity nullified by his denying our Consciousness of Power — Distinction between Sensation and Perception — Of what is the Mind Conscious ? — Arnauld's Views — Difficulties — Distinction between our Perception of the Primary and the Secondary Properties of Bodies — Our Knowledge of Locality and of Local Corporeal Affections acquired through Experience.

XXXVII. Secondary Properties of Physical Bodies...

They are Mental Affections— The Quality, as distinguished from the Forms, of our Sensations — Confusion from not observing this Distinction— Reid denies the Efficacy of Physical Agency — How far our Sensations are Significant, and how far Arbitrary and Meaningless. XXXVIII. Primary Properties of Physical Bodies

Difficulty felt by Reid and Stew^art regarding Power, Cause and Effect, and Physical Efficiency — Reid neglects the Distinction between the Quality of our Sensations and the Forms in which they are presented — Hamilton Enigmatical — Stewart endorses Reid's Views on Perception— His Mysterious Explanation of Perception — Locke.

CONTENTS. XV

CHAP. PAGE

XXXIX. Philosophy Seeks an Exhaustive Theory of the

Three Questions Regarding Power — The Mental and Physiological Phenomena which accompany Physical Effort — Opinions of Hume, Reid, Brown, Mill, Hamilton, and Eraser — Our Knowledge extends not to the Ultimate Cause of Power — The Perception of the Active Properties of Matter is to us the Perception of Power — Reid and Hamilton's Views on this point Inconsistent with their Realism — Our Perception of External Resistance Mediate, not Direct — Our Knowledge of certain Properties of Matter the result of entirely different Physiological and Mental Processes — Inoperative Mental Effort — The Muscular Sensations.

XLI. Atomic Bodies — Free, An-atomic, or Transmissible

Perceptive Knowledge Acquired in three different ways — Distinction between Atomic Bodies and Free Transmissible Power — the Transmission of Force — The Theory of Force a proof of an Immaterial but Efficient Agent in Nature — The Conclusions to which this leads — Philosophical Statement of the Nature of Perception as a Connection of the Mind with the Sustainer of Nature's Energies — Additional Examples of Free Physical Force — Its Apparent Velocity of Transit — What this means — External Power known only by its Effects — Its Laws — Its Course.

Recent Advances in our Knowledge — the Functions of the Nervous and Muscular Systems — Motor Nerve Fibres — Striated Muscular Fibres — Muscular Fibrillae — The Nature of Muscular Contraction — The Mental Principle as an Agent — Power obtained by Decomposition of Animal Tissues — The Physical Function of the Mind, of the Brain, of the Muscles, in Voluntary Motion.

XLIII. Concluding Sukvey ... ... ... ... 535

ERRATA.

Page 5, line 4, for "respect that they are both" read "in as far as they are." „ 52, line 15, for "pervading and animating it* ?r<w? " pervaded and animated by it."

LIST OF ILLUSTRATIONS.

SOUND.

Eleven Diagrams illustrating the Laws of Sound The Human Ear enlarged ...

The Cochlea

LIGHT.

Diagram illustrating Newton's Theory of Light ... ... ... I39

Four Diagrams explaining Theory of Vision ... .. ... 150, 151, 152

Four Diagrams on Refraction ... ... ... ... 155, 157, 160

Horizontal Section of the Right Eye ... ... ... ... 176

Diagram showing cause of Short and Long Sight ... ... ... 180

Magnified Section of the Retina ... ... ... ... ... 184

Diagram of Retinal Image and Fibres of Optic Nerve ... ... ... 185

Transverse Section of the Retinal Cones ... ... ... ... 186

Diagrams showing Four Methods of Magnifying Objects ... 190-192 Diagrams explaining Single and Double Vision and Corresponding Retinal

Visual Direction illustrated ... ... ... ... ... 239

Diagram of Pure and Complementary Colours ... ... ... ... 245

CENTRAL ORGANS OF SENSE AND ORGANS OF VOLUNTARY MOTION.

Section of the Spinal Cord, showing the Nerves of Sensation and of Motion 301

Typical Brains of Vertebrate Animals ... ... ... ... 306

Section of Right Half of the Brain ... ... ... ... ... 3^9

Central Parts of the Brain, showing the Cranial Nerves ... ... 321

Magnified Nerve Fibrils ... ... ... ... ... .. 525

Ultimate Muscular Fibrillae magnified 1200 diameters ... ... ... 526

Muscular Fibre breaking up ... ... ... ... ■ • • 5 2 7

Part I. Matter— Organisation— Animal Life, Etc

PHYSICS AND PHILOSOPHY OF THE SENSES.

Chapter I

MATTER— ITS ATOMS— THEIR ENERGIES. POWER— OUR CONCEPTIONS OF IT.

The animal alone is the subject of sensation, self-consciousness, and intelligence; but the philosophical treatment of THE Senses involves a study, we may say, of all nature. It excludes no phenomenon, mental or physical, which, from possessing any distinctive character, may help to throw light on the subject. The physical world being the object of sense perception, forms thus a principal part of the problem to be examined, and the nature of matter becomes a first object of inquiry.

It is evidently important that the attempt be made to arrive at some more or less distinct conception regarding that substance of which all things visible and tangible consist, and even if it should be the case that we may not succeed in forming any perfectly clear conception of the true nature of this great basis of the physical world, it will, at least, be a point gained, if we help the reader to discover wherein the difficulty lies.

It will be admitted by all but idealists, who deny the existence of an external world, that the senses, and our active powers, reveal to us a vast variety of curious and instructive phenomena ; it is, therefore, reasonable to expect that, if we examine these aright, some knowledge of a reliable kind will be reached.

Our examination, of course, must not be restricted to a mere noting of the forms and motions and colours of physical objects, as these are given by our passive sensations. We must look more closely than this, and, at the same time, with ' the utmost simplicity, in order that we may discover w^hat are our real ideas of matter. If we are to obtain any light, it is evident it can only be by a faithful exercise both of our observing and of our reflecting faculties. If we have hitherto failed, or arrived at false conclusions, it is just as probable that this may have arisen from want of simplicity, as from lack of ability or effort.

Every variety of physical substance is described and recognised only by its qualities or properties ; and if we reflect, we shall find that the possession of a quality or property implies the possession of some distinctive power, which enables the object to affect us, the observers, or to affect other bodies which may be brought within its influence.

Had the object no power of any kind, it is clear, humanly and rationally speaking, that it would be of no service in creation, and we could not even become conscious of its existence by the senses, or by any known means ; for we know of the existence of physical bodies only by their having the power of affecting us.

The simplest inorganic mass, and the minutest particle of what is called matter, may be conceived, in this regard, in one or other of two ways ; thus, either, first, we may regard it as possessing certain powers of action, peculiar to itself, and which originate necessarily from some property inherent in the particular substance, and which we may say is of its essence; or, otherwise, viewing the individual substance as a part or portion of a vast, skilfully-designed, and compacted physical system, we may regard each individual part as the mere instrumental cause of the action we observe it performing. In this latter case we regard the manifestation of power as being exerted 07tly in connection with the different substances or objects^ while the source of the power remains unseen and unknown.

To both of these theories, in respect that they are both founded on the existence of an independent entity, called matter, we have insuperable objections on account of the inconsistencies which each of them entails ; and this we shall endeavour to show. And we hope the reader will excuse us if in our remarks we necessarily become a little metaphysical.

Science has been unable to account for the existence of property or power in connection with any physical substance, or for the different properties of different substances.

It may be suggested by those who are in the habit of only regarding outward forms and appearances, that if chemists could ascertain the peculiarities which distinguish the elementary atoms of different known substances, they would then be enabled to explain the reason of their exhibiting their various distinctive qualities. Thus it may be supposed that there is something distinctive in the sizes, forms, or weights of the different atoms which determines their modes of action ; or that there may be something \rv the position of their attracting and repelling poles. And this, so far as it goes, is likely enough, for form and rule and method are strictly observed in all physical arrangements. But such suggestions as these do not touch the difficulty, they do not account for the energy or power which we conceive to be in the substance.

Perhaps, however, the more general, loose, and popular notion regarding matter is, that elementary substances, having different properties, must be essentially different substances. This is a suggestion objectionable on every ground, and yet it is the prevailing theory. For purposes of convenience, we are taught that there are, at present, at least some sixty-five different known elementary bodies ; and nearly every year is adding to their number. It may be necessary for the purpose of teaching chemical science, that we countenance this language ;

but it is evidently intended to have reference only to outward manifestations, and not to interfere with the question of philosophy, which seeks to know the ultimate essence of things. It is, however, as we have said, philosophically objectionable: in the first place, because it multiplies entities beyond all rational bounds. While Spinosa, who so long pondered over the subject of being, could not bring himself to see the possibility of more than one substance, and that a spiritual one, to wit, the One Infinite Being, they who do not reflect are ready to accept fifty, sixty, seventy, or, if need be, a hundred essentially separate substances.

But, again, it is objectionable because the assertion that elementary bodies are essentially different entities is an assertion possessing no distinct meaning. When examined into, we find it impossible to bring any plain meaning out of it, except by giving first a description of the more external forms and colours of the bodies, and then of what marks their more important properties ; as, for instance, their various amounts of solidity, tenacity, ductility, etc., and the various chemical relations which they affect. But these, and such as these descriptions, it is evident afford no insight into the essential nature of substances, seeing they have reference, as we have said, only to outward manifestations. In fact, in all such attempts we do nothing but describe in a circle, and never get an inch nearer the definition of the object w^e have in view. For instance, we explain one unknown body only by means of another equally unknown ; thus, by solidity we mean no more than that one body resists the entrance of another body ; by chemical affinity, that one unknown body acts so and so upon another unknown body. This is all our attempts at explanation enable us to say. The moment we attempt to define what we mean by substance, or essence, or elementary body, the mind finds itself fixed as in a metaphysical vice, which arrests all further thought and motion.

Seeing, then, we can form no conception of the nature of matter or of tdti7nate being, the next question, and one of nearly equal importance, is this : Whence come those powers of action which we observe in physical bodies ? How do physical bodies happen to possess a power of acting in this way, or in that way, or in any way implying energy ? If we can answer this question to our own satisfaction, we gain an important point, for we settle to the satisfying of reason one of the chief questions of philosophy.

But can this question be answered ? It seems to us that it not only can, but that it must be answered, for it is constantly before us, pressing for an answer; and though in answering it we are conscious that we are doing no more than forming an inferential judgment upon the data before us, and which falls short of what philosophers call a demonstration, yet this is clearly no reason for man sinking down into apathy and indifference, and professing an out-and-out philosophy of ignorance and scepticism. On the contrary, as moral beings, gifted with high faculties, we hold that man is called to repose with confidence upon the verdict of his mind, when it is honestly exercised on subjects such as these. Having a judging faculty given him, with full freedom to exercise it, we maintain he has good ground to trust this high and ruling faculty, when it is honestly and properly exercised.' The human mind, be it observed, forms its conclusions not by chance or hap-hazard, but by rule. Its decisions are, therefore, not uncertain and variable. Like nature, its laws are fixed and immutable on all such subjects as lie properly within its province, and therefore is it that they who possess such a faculty are bound to repose trust in it.

If, indeed, man seeks knowledge beyond his range, he must expect to suffer disappointment. As, for instance, when he tries to define the nature of the One Being, or of any being, he must fail, for the human mind can form no conception of being in its abstract nature. But if we are satisfied to know that wisdom and power are exhibited in the world, and therefore that wisdom and power exist, and are the operative elements in nature, this seems to us to be a judgment which lies quite within the natural bounds of man's faculties ; for, far from experiencing any ineffectual straining of the mind to form such a conception, we find a difficulty in excluding it from the mind. There is no arrogance, therefore, in our forming conclusions on this subject ; on the contrary, it presents itself to us as a necessity of our being to do so ! Neither, if we keep the whole facts of the case before us, does there appear any possibility of a false judgment. No doubt the materialist proclaims an eternal Cosmos ; but the more thoughtful mind replies. Yes ; if this Cosmos possesses all the elements, moral and physical, which we see existing in the world and in our own minds, and especially iiitelligence and power, working ever forward to definite ends.

Starting, then, with the preliminary conviction of our incompetence to understand the nature of ultimate being or substance, but retaining a due estimate of the powers of reason within the legitimate field of observation and reflection, let us quit such considerations as have been occupying us, in order to make some such simple remarks on external nature as we cannot expect to be disputed by any one.

While there are a great variety of properties observable both in natural and in artificial substances, there are certain properties which seem common to them all, and without which we can not conceive a physical world to exist ; such are, first, the occupying of space ; then solidity, or the power of resisting compression ; then tenacity, or the power of resisting the separation of their parts ; then gravity and chemical affinity, both of which last are attracting forces ; then mojuenttinty or the power we discover to exist in moving bodies.

It is a very common and not very unnatural opinion, which many people form, that matter is inert. They do not deny that chemists are in the habit of exhibiting many dazzling experiments in their lecture rooms, which show the energy of matter. But their acids and alkalis and galvanic apparatus are not to be found ready made in nature. What is seen on the surface of the earth seems to them entirelyloose, inoperative, and inert — a mixture of clay and sand and lime and splinters of stone, and these ingredients seem dead and inert enough. We require to be reminded that these substances, inactive as they seem to be, have all in their day exhibited abundant energy and brilliancy ; and that to reduce them to their present state has required an incalculable amount both of chemical and mechanical power. The clay, for instance, which is the most plastic of them all, when it was combined in its molecular state with silica, in felspar, basalt, and other volcanic rocks, maintained its connection with them so firmly that it required the mechanical action of millions of years to grind them asunder. But more than this, these aluminous particles, though they now seem to be so inert, are still exerting, individually, inconceivable power: each molecule, as we know, is a compound of oxygen and a bright metal, and these cling so strongly together, that chemical science has only recently learnt how to separate them. And the same may be said of the lime, silica, and other substances which constitute the covering of the earth. If by some unknown microscopic or other power the eye of sense, or the eye of reason, could examine any single particle of what lies under our feet, we would discover that it consisted not of dirt, a term opprobriously bestowed on it, but of millions of mysterious and imperishable orbs, bound together into patterns and systems so symmetrical that they would surprise us no less by the force that binds them together than by the geometrical precision of their relationships. A friend, on one occasion remarked to us, rather warmly, that he never could perceive any energy in matter. If it be so, we replied, take up that paving stone in your arms, and try whether it or you have most energy ; or try to crumble it between your finger and thumb, and be convinced, once for all, of the extent of some of its powers.

Modern chemistry is founded on the doctrine of the atomic constitution of matter. This is an inference or theory delO MATTER AND ITS ATOMS.

duced from the circumstance that substances, having mutual affinities, unite in definite proportions. Let us attend to the law, as illustrated by some of the best known and most important elements — oxygen, nitrogen, and carbon.

One atom of nitrogen in chemical connection with one atom of oxygen forms protoxide of nitrogen, or laughing gas. It may be represented thus, —

(where the atom of nitrogen is distinguished by a cross). We may remark that in the various gaseous compounds we mention, we shall represent the atoms as standing apart, because in aeriform bodies the constituent atoms must always be far apart. Were we, however, to preserve the true relative distances, the page would probably not contain the figure of a single molecule of any one of these gases, if the atoms were given on the scale we here represent.