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ThoSe who have spolien of Species as spirituat, do not uSe that word as it is used of God or the Soul: they mean impercepti ble. But Such a use of the word is Confusing. The species resembles that ofwhicli it is the image ; it is the resore materiai. It passes througha materiai medium; is there re materiai. It produces materialreSulis ; being related to that result as the incomplete to the complete, as the embryo to the fulty formed animal. Species are indeed invisibie, except it be by accident; when an opaque body interceptSthe ray of light or Colour, we become sen Sible of it. Or again, Weakness Of sight may malae us sensibi e of the passage of the ray, thoughwe Should not be So other i se. The sanie principie applies to objectSperceived by the other Senses. It may be asked, How is it that three candieS placed near a Smali opening appear as three on the otherside Θ There is true mixture of the species in this CaSe, for one illumination resulis. Stili is the eye be applied to the opera ing, thedistinction of the principat rays proceed ing Dom each will mahettself felt . 43-46S EVENTH DISTINCTION 47 53
Importance of the disserenue in densi ty bet ween crystalline flensiano vitreotas. Is the vitreous were of the Same densi ty RS the len S, the rayS Pa Ssing frona the lalter to the sortiter would follow their
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The action of the eye on the object Seen. Lilae every other objeCt, the eye radiates SpeCieS. But are these radiations, or those of themwhich touch the objeci, necessary to vision 8 Aristotie's rem arks in his treati se on Generation Seem to potnt to this. Ptolemy and subsequent optical writers have asserteo it. Plato maintained that sensation was wholly active ; the Stoics that it was wholly passive. Aristolle talaes a medium course. Vision is in faci not merely
Nor do Athagen, Avicenna, and Averroes contest this vi e N. Theyonly protest against the notion that the eye emit S Some materialsubstance to the thing seen. A passage in Aristolle De Anima, ii. II, has been understood as though he looked on sensation as whollypassive. But it was only by way of protest against the Platonisis, whΟwent too far in the opposite direction ...... 50-52
As ali objecis in nature complete their action by emitting their own special forceS, So cloeS the eye generate rays Dona itSelf. These meet the rays proCeeding stom the object to the eye, and facilitate thepassage of these lalter. As they are of different nature no confusionarises Dorn their meeting 52-53 EI GHTH DISTINCTIO N. FOUR OF THE CONDITIONS REQUIREDFoR UISION 54 61
CHAPTER I. The first condition is Light. Without light, colour ei ther sa) does
not exist, or b) Sends out no emanations, or scin What emanations it produces do not affect vision. The third of these explanations is thetrue one. The second Condition is, Distance of the objeci. An objectplaceo in immediate contact with the organ of sense is not perceived: for the organ acts by emitting force hom iis eis in to the medium. This is the Case even with the senses of touch and taste. The medium for these is the flesti and shin surrounding the sense
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The third condition is the Position of the object in respect of theeye. They must be connected by Straight lines ei ther direct, refracted, or reflected. Why is this the case with vision, and not so in the Case of hearing, or of the sensation of heat Z Aristolle here fatisus. The dissicut ty is obvious. A man Can hear hi S Own voice : whycannot he see his own face 8 It would seem that Sohand generates Something more than species of Sound : it must generate Sound it self The trembling of the air sol lowing on that trembling of the objectstruck whicli constitutes soland , is propagat ed in ali directions, andis the refore perceived as soland everywhere. In the Same way anodorous body not meret y sen d s out Species, but Subile materialparticles, which being diffused through the air them selves emanate in every direction. It has been at ready explaineo that there are certain qualities capable os completing their Species, by generatingfresti fources of radiation. Fire is One of those .... 56-58CHAPTER III.
The Durth condition os vision is that the object be of sui table Magnitude. Each potnt of the object must be represented on thesuriace of the lens. Anci although that sursace, like ali others, is inii nitely divisibie, yet, for purposes of sensation, the divi Sion musinoi go so far that the potnis beco me undistinguis habie. Again, theobject must not be too large. What are the limits 8 I f the extremeangle of vision be a right angie, an eye placeo at the earlli's Centre Would see one Murth part of the heavens ; at the earlli 's furface Something leSS, as geometry shows. The view that the limit of theangle of vision must be a right angle resis on no Sound baSis, ei ther of theory or experiment. Examination of the structure of the eyefurnishes geometrical proof that it is tess than a right angle. Andexperiment malaeS this Certain ....... 58-61
The fifth condition os vision is that the Densi ty of the objectshali exceed that of the air and of the heaven. This is why we
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The fixili condition os vision is Rari ty of the Medium. It is objected that flame is rarer than air, and 3 et that a flame placedbetween the objeci and the eye impedes vision. But it is a
mi Stake to suppo se that flame is rarer; and Alli agen, who has been quoted as saying this, is misinterpreted. Whether a lynx Sees through a wali or not, the human eye which we are here considering certainly does not. On the other hand, we are not to SuppOSea vacuum belween heaven and earth. Radiation os species Wouldbe impossibie in that case. A vacuum is a mere mathemati Cal
The sevenili Condition is Time. Time, as Aristolle Shows, is ne ededfor an aCt os memOxy: muCh more sor an act of sensation. It hasbeen maintained that radiations frona the objeci and the eye talae place in an indivisibi e instant of time. For is not, then the parti Cles of time, however smali, would form a percepti ble aggregate while ther diation passes from the east of the heavens to the west. Thisvi eiu is held by ali but Athagen, and by him is contested on insum cient ground s. Some however of his arguments against it arefound, resting as they do on Aristotie's doctrine that finite energyrequires time for iis operation, the time being inversely as theenergy. SuppoSO the energy infinite, then, and then only, doeSthe time beeome gero. The notion that the ray is spirituat not
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materiai, and there re not amenable to physical laws, has beenalready disposed os. It is materiai, and as Such Cannot be in severat place S at the fame time 68 Tl
It may be further observed that the time occupied by a luminous radiation may be so smali a S to be imperceptibi e to our Senses evenwhen the distance traversed is very great. Aristotie's supposed dental os it was a dental of the view of Empedocles that a corporealhody Was transmitted across Space. But the luminous radiation is not a body, but a forna continuat ly rene ving it Self ont of the particles of the medium, as it traveis. It is true, as Aristolle Says, that thereis a disserence bet ween the transmission os light and that os foundand other Sen Sory impressions. With sound there are three distinctdisplacements of material particles, vi g. motion Os the body stria Ck, tremor of the air, and rarefaCtion of the a tr. Nothing of this hindialies place With light, sor though light mahes heat and rarefies theatri yet it is transmitted through celestiat SpaCeS where rarefactionand heat-production is impossibie. In the transmission os odo urs, again, there are special differen ces. But ali need time: though notthe Same time. V e see the Strohe of a distant hammer bes ore weliear it. Lighining reaches us belare thunder. The eighth condition is a healthy state of the eye. A final condition, relating to the axis of vi Sion, Will be Spoken os asterwards
We may now inquire With greater precision of at matters visiontakes Cogni Zance, What degree of certainty it brings, and to what errors limiting ourselves to direct vision in it is exposed. Knowledgeis et ther gi ven to his directly and immediat ely by Sense, or in directly and fer accidens. Supposing the eight conditions described to bepresent in just degree, the eye Will perceive the twenty- two qualities previ ousty Doted, with or without the hel 9 of the other spe lal senses, and of Sensus Communis and asinatio spp. 4-53. By qualities in directly sensibi e I mean those whicli Come under our Cogni Zan Cethrough sensation, but whicli are appreciated by other faculties, asjudgement; as When a lamb seeing an object of the forna and colouros a Wolf, knows it to be an enemy. When I See a man, I See an animaled objeci and a substance; thus in an indirect way SubStance may be spolien of as Sensi ble. Again, the Sen Sations peculiar to one
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SPECIAL CONSIDERATION OF DIRECT VISION . . . 83-l29FIRST DISTINCTION. ON VISION AS DETERMINED BY THESTRUCTU RE OF THE EVE 83 91
osten prolong this channel by holding the hollow of the hand
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Doubie vision resulis Dona disturbances in oCular StruCture S. In the normal condition of the eo es, the objec t Seen apponi S single.
Is the eyes are congenital ly or forci bly displaCed, this uni ty of the object is lost. Extreme heat or Colo may produce this result; oranger, infirmity, or drint . Duplici ty of Vision may depend either
portion of the lens. Another cause of duplici ty has been Suggested. The radiations issuing srom the eye may be so fe eble as to be stoppednot merely by the object of vision, but by Some one of the radiations Dona this objeci. It is state d also that in sonae cases the eye has had two pupil S, though I have not myself seen thi S . . . 88-91SECOND DISTINCTION. ON RADIATIONS FROM THE OBJECTAND FROM THE EVE, AS AFFECTING DIRECT UISION . . 92-99
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Dom the eye Rre Strong enough to penetrale it When near; not Sowhen distant. V e must conceive the pencit of rays Dona the objectand that Dom the eye as having a common axis drawn through the Various ocular centres; this Central line being specialty potent invision. The poliat at whi Ch it stri hes the object is seen with CleariaeSS ; adjacent poliat S at So, in proportion to their proximi ty. The axis of the eye is directed successively to each potnt in the objeci. The eyes aci in concert, their axis being directed to the Same
When one of the eyes is pushed by the finger froni iis properposition, the angle of Vision Ceases to be the fame for both of them, and the objeci appears double. So too when the eyes are fixed on agi ven potnt, an object placed belween that poliat and the eye, orbeyond that potni, will appear double. It is easy to demonstrate this by eXperiment 94 95CHAPTER III.
Let iis consider further the three modes of perception, through SenSe, through recollection, and through argument, in referenCe to the eight constitions os vision. Light and colour are appreciated through senseonly : and this without error, so long as the right conditions os visionare fulfilled without excess or defeci. Stai light is not seen by daybecause of excess of solar light. Diministi the lalter, as by descend ingto the bottona of a well, and the Stars are seen. The Mithy Way,
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consisting os a multitude os smali stars Clustered, produces theerroneous impression of a Continuo us light; this is clue only todistance. This luminous impression is Cata Sed by the passage of rays Dona these smali stars through the sphere of fire ; the medium in the Stellar sp Ces Would be too rare that of the sphere of air tooden Se. We have also to talae into account the refraction os rays in the Sphere of fire ; but of this aster vards. The light of dawn rai sesa question. Why do we not See it earlier Θ why, Since out si de theearth's shadow the whole sky is illumined, does the shy appear dari to us t The reply is, first, that the Spheres of heaven are very distant; also that the heaven, apari froni the dense bodies Containedin it, is too rare to have fixed light; light passes through it without affecting the eye. When the Stan's rab A touch that portion os atri 'hicli is comparatively near to his, this illuminated air beeomes percepti ble. It may happen that a luminous body of smali magnitude appears far larger than it is on account of rapid motion . Sparks Dona a fire produce this effeci. Shooting stars are probably bo dies of smali magnitude. Colour, like light, is apprehended by pure SenSe ;and correct apprehen Sion os it clepenos On the Conditions before mentioned. Extreme transparen y of the objeci, great intens ity of the Colour, too great remoteneSs of it. rapio succession os different colours, Will intersere with accurate perception .... 99-104
CHAPTER II. Os Perception through knowledge recollection). Under this headare to be classed distinctions os characters, whether general or speciat, in visibi e things. The moon's light ouisi de the earthys shadow is clear and white, in the up per part of the shadow red, in the lower part is invisible. These differen es, though perceived by si glit, beComelinown to us Only after repented ObServation. The explanation is noteasy. The moon's light, like that of other stars, Comes Dona the Sun. When the moon is in conjunction. the hin drance to illumination of the part of the moon turn ed to us is the moon itSeis; the solar rays diverge widely on ei ther fide, and do not rench uS, CVen Se Condari ly by dispersion. But when the inoon is in the earlli's shado , the earthbeing distant frona the moon, the moon is near the Vertex of the Cone
and accidental Solar rays entering with in it produce the reddish light seen in a lunar eclipse. The degree of this light will vary with the distance of the moon froni the earth at the time of eclipse, whi Ch maybe greater or leSS. Another problem, rat Sing a similar dissicut ty, occurs with Colour. Is we loOh through a very thin piece of parti coloured Cloth at an object os gi ven colour, we may See that Colour is the apertures in the web are large enough ; is other i se, we See a miXed colour. The quali ty of the medium urili modisy the result.
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fuSed With them . . . . . . . . .. 104 106
Os Perception through reasoning. Many exam pleS Can be givenof this. The most stri Ling is our perception of distance. An objectmay be So di Stant, as to subtend so Amali an angle in the eye that vision ceases. Shori os that limit, the degree of distance is determined by a continuous series of Objecis belween the objeci and theeye. In A flat country we have no means of judging the height of the cloud S, which we Can do when ive see them on the fides orsummits of mounta in s. Clouds Would appear to be of no greatheight, though, as We know froin the fact of twilight, exhalations other than Clouos may rise fifty-one miles. Such exhalations arenot aqueous: being dry they retain the stan'S heat better, and thus rise higher. J For jud ging, then, os distance, We must haVe an intermediate Series of objecis, each of which shali be appreci able by theeye with sussicient accuraCy. These limit S are SOon OXCeeded. A line of troes appears Continuous, though there may be a great interval bet ween each of them. So planeis seem to be in the sanae Surface as fixed stars. though the differen Ce of remoteneSs is immense. So an equi laterat figure of many fides beComes Ata distance undistinguishable from a circle. A circle may be taliensor a straight line, a Sphere for a plane figure. When a circle is heldsidewayS besore the eye, the part nearer to the eye will be recogni redas nearer is the distance is moderate : is it is very far ossi the differen eos distances in the potnis bears so Smali a proportion to the wholeas not to be recogniged. Thus it is that, when the moon is in herfrsi or third quarter, the circular line defining the light pari froin thedarla appears as a stra ight line. So too the sun and inoon Seem tous flat, though they are spherical 106-108 CHAPTER IU. All this is exempli fied in the study of the Moon's Phases. The